Cutting and Handling ACM Panels
The panels come with a protective film that is pretty tough. I can load them into the back of a pickup without worry of them being scratched. This film peels off easily. I usually leave it on until I've cut the panel and am ready to start sanding.
The panels can be cut a number of ways, but the best way I've found is on a table saw with a common 32 tooth carbide blade. The Aluminum isn't that much of a challenge for a saw blade as the aluminum is only about as thick as ones thumbnail. This method leaves a nice edge with a small burr that can be sanded easily with a few passes with 150 grit paper on a sanding block.
Other methods, such as cutting with a box knife, leave the edge with a bit of a ridge that is much harder to sand down.
In preparation for priming I sand with a 320 grit sand paper on a small cheap orbital sander. I've used Garnet and Aluminum Oxide papers they both work fine. The polyester paint sands quite nicely, making a free sanding dust without gumming up on the sandpaper. The objective here is to sand the surface thoroughly without sanding through the paint and into the aluminum. Make sure there are no glossy spots. I've used many different papers, my best results were with 320 grit paper and a very light pressure
I wash the panels in water and dry them to eliminate the sanding dust. Just before I prime them, I wipe each with a little rubbing alcohol to clear any surface oils and they are ready to prime.
I use DTM Bonding Primer B66 A 50 by Sherwin Williams. It's a water based Distributed Acrylic Base which provides strong adhesion and is especially designed for priming over pre-finished metal siding such as the polyester polymers used on ACM's and Dibond. The B66 A 50 includes a small amount of marble dust that provides a nice tooth for adherence of Gesso or other top coats to follow.
Especially note that there are two kinds of DTM primers that Sherwin Williams sells. Be sure to buy the Bonding Primer B66 A 50 which is intended to have a top coat application and leaves a very nice toothy surface.
The DTM primer provides a very strong bond to the ACM panel's polyester coating and is the primer recommended by Golden and several technical sources I've found when Dibond was first used for panels. Now it is generally recognized that one can apply Acrylic Gesso directly to the sanded polyester panel surface. I still prefer the Sherwin Williams DTM bonding primer as a first coat due to it's smooth easy application and the toothy high surface area it provides for adhesion. See below the section on "Selecting a Primer" for more details.
DTM Primer Application
I apply the primer with a 6" white, high density, micro foam roller that I buy from Lowes. It's the one that they recommend for painting doors and cabinets. I apply 3 thin coats of primer to get complete coverage. No sanding is required between coats.
I thin the Primer right out of the can, adding no more than 5% water. I also have a plant mister nearby to add additional water if needed.
I use a large palette and pour a couple of tablespoons of the primer out and work the primer into the roller very thoroughly. I do not load the roller heavily as I'm applying thin coats and trying to keep the texture very smooth. If you wish to build up the texture increase the loading and reduce the water to make the mixture thicker. I prefer a smoother texture, similar to a quadruple primed linen so I keep the coats thin. Experiment until you find your preferred texture.
I load the roller on the palette then work it into the panel thoroughly to insure coverage and then I make several light passes to insure I've left no ridges or furrows and the result is a very smooth, thin layer. I allow approx. 2-3 hours between coats, more if I have time.
I try always to load the roller on the palette. I never pour the primer onto the panel and roll it out from there. If you pour primer directly onto the panel it may loosen and lift the previous coat of primer. Also the results are more consistent if you are applying with a roller that's always loaded the same.
Roller and palette clean up with water is easy.
I've used only two kinds of Gesso on panels Golden Gesso and Liquitex. Both worked well. Liquitex is a little thinner and so one doesn't have to thin it as much, otherwise they seem to perform the same.
Today it seems to be a common practice for some artists to skip the primer and apply acrylic gesso directly to the sanded panel. Golden recommended the DTM Bonding Primer when Dibond first began to replace Masonite and now they are saying that their Golden Gesso can be applied directly without primer.
Using Liquitex Gesso I squirt out a little onto a large tile palette. I thin it only slightly with a squirt of water from a plant mister and very thoroughly distribute the gesso into the roller, so it will apply evenly. I very lightly load the roller.
Liquitex has some advantages if only for the reason that it is less messy to get out of the container and onto the palette, but Golden Gesso also works well. Golden is a little thicker and requires a bit more water when thinning.
I apply 3-4 thin coats which provides quite a smooth surface that requires no sanding between coats. If you want more of a textured surface just load your roller a little more or thin the gesso less. Keep trying until until you find the texture you prefer.
Even though the first coat is thin there will be a lot of meringue like peaks that betray a smooth surface. Those will dissipate a bit in the first few minutes but not completely. After the first coat has dried for about 2 min then go back over with the roller (mostly dry not loaded). With no pressure on the roller, lightly roll the surface again breaking down the roughness and smoothing the surface.
I use three to four coats to get a smooth surface that covers completely. And the best part; no sanding is required between coats or after the last coat. The only sanding I do is when I sand the panel in preparation for the primer. The thickness of the application of course determines the drying time between coats but it dries pretty fast.
I will say that the smooth, no sanding required, surface is not always a sure thing. My last batch of panels were much rougher and I found they needed to be sanded a little. Sanding Gesso is not a pleasant job as the paper gums up pretty badly and wet sanding is a mess.
Clean up..... If I'm gessoing or priming several times in a day, or even the next day I wrap my roller in a plastic WalMart sack to keep it from drying out between uses. My coats are pretty thin, I wait about 2-3 hours between coats. Like the primer I do not pour out any gesso on the panel. I work the gesso into the roller on the palette then roll the next coat on. Pouring the gesso on the panel could soften and lift previous layers.
After much review with various Technical Departments and experienced ACM users as well as other "Best Practices" sources, I'm pretty comfortable with this system. It makes a smooth surface that's not extremely absorbent and not too slick. I'm trying for a smooth slightly pebbled texture. Adjust the method and find your technique for the texture you most enjoy.
Applying Oil Ground to ACM Panels
For those wanting to paint on an Oil Ground surface the following method applies. I use the same surface primer and application technique that I use for the Gesso above.
Choosing an Oil Based Primer
I tend to go first to the manufacturers and commercial users recommendations for advice. The fabrication Manual from Alcan/3A Composites and other ACM manufacturers recommend that the best direct adhesion occurs after lightly scuff-sanding the painted polyester surface, followed by wiping with isopropyl alcohol, and then application of properly selected paints/primers. They also caution not to sand through the coating to the aluminum surface.
Several Acrylic Primers are mentioned among the list of “suitable primers”. I found it significant that oil based paints/primers ARE NOT on this Manufacturer's list of approved primers. There are some oil based paints and inks that are recommended for direct application. But these are inks and sign paints specificly made for sign painting and large scale inkjet printers. A broad majority of products recommended as primers were all acrylic based primers citing adhesion to the polyester coating as the issue.
Two bonding primers seem to be mentioned most often in the users sites I've been on. Sherwin Williams DTM Bonding Primer B66 A 50 and Rustolium XIM/UMA Bonding Primer, both are commercially available primers. Both are designed to be primers suitable for polyester coatings as found on ACM panels and are often used in other hard to coat surfaces. I've been using DTM Bonding Primer, it has a history of many successful, interior AND EXTERIOR applications. This does not mean that other similar primers could not work as well, but these two have a successful track record for this kind of application and are recommended by the panel manufacturers. The DTM is recognized as having more of a toothy texture providing a larger and stronger surface area for adhesion of the top coat.
I've found recently there are several manufacturer's that are recommending some of their Oil Based Grounds to be acceptable for application directly to a sanded ACM panel. Gamblin and Rublev are two that I'm aware of. There are also some of the prepared ACM panels that can be purchased on line that have Oil Based Grounds applied directly to the ACM panel.
Although it adds one extra step in the process, I still choose to use the Acrylic Bonding Primer and feel it is the best choice as a first coat for the reasons stated above. It does seem that some very reputable manufacturers are discontinuing the practice of Acrylic Primers under the Oil Based and Acrylic Gesso Grounds.
See the following article from Golden Artists Paints
Choosing an Oil Ground
There are several choices to be made when it comes to choosing an Oil Ground.
There are many strong opinions that seem to separate into different camps.
----Titanium Alkyd Oil Based
----Titanium Linseed Oil Based
----Lead Alkyd Based
----Lead Linseed Oil Based
And then there's the "some Zinc is ok" or a little Zinc is OK and "Zinc is NOT ok in any amount" camps.
And then as a practical matter, price and availability is another determining factor as to which gound one ends up using.
I'll try not to enter into the opinion fracas, and just discuss the ones I've tried.
Lead Linseed Oil Based (no Zinc)
by Rublev (Natural Pigments)
I haven't painted on it yet but application was easy. I rolled it on using the same palette and a roller similar to the roller I used for the primer.
No thinning was required. I rolled it on just out of the can. Odor might be a consideration as there was a definite strong linseed oil smell. Let's just say it was mentioned by the family. I was painting on the back porch.
Drying time was longer than the Gessos of course. I waited 48 - 72 hours before I applied the second coat. Only two coats were required. The finished texture looks very similar to the Titanium Alkyd Based Primer by Winsor and Newton. I'm looking forward to painting on it.
Titanium Alkyd Based Primer (no Zinc)
by Winsor and Newton
I have painted on this one several times and it's a nice ground. A finish that's not too absorbent and not too slick.
I applied the second coat in 24 hours. Odor... there is a noticeable OMS ordor but not bad. I would apply the Oil Based Grounds somewhere away from ones living areas. At my house priming and finishing panels on the kitchen table is discouraged.
I thin this one with 3 to 4% by wt with OMS. It rolls on nicely and gives a reliable finished texture without a lot of fuss. Two coats is enough. And there's no sanding... The texture is smooth but slightly pebbled similar to a good multi primed linen.
Of note here.... This product is made by Winsor and Newton one of the brands under the umbrella of Colart, a European company. In Europe, the nomenclature is a little different. They use the term "Primer" when we would say "Ground". In some circles Primers are a little different than Grounds but I get confused when we're mixing terms and continents! My friend in the UK says this Winsor and Newton Primer is what we in the USA would call a Ground!
I "can say" that it is difficult to have a phone conversation with someone in Winsor and Newton's technical department ! In comparison I've found Gamblin/Golden and Rublev communications to be quite responsive.
I'm hopeful I can provide more info on other grounds in the near future!
Bonding Canvas to ACM Panels
Bonding Canvas using Natural Pigments Beva Film
--- sand and wash the panel as described in the method above.
--- mist the panel with alcohol and wipe dry and clean from grease and dust
I"ve used Beva Film as my adhesive from Natural Pigments... it's a heat activated adhesive film that comes in a roll. It's a thin layer of adhesive applied to a clear plactic sheet and backed with another layer of opaque plastic.
I cut the film to the size of the panel plus about a quarter of an inch on each side. Leave just enough overhang so that you insure 100% coverage. But too much overhang will get messy when the glue melts in the press.
Then I peel the opaque plastic sheet off leaving the adhesive sticking to the clear plastic film. I lay the plastic film (glue side down) on to the panel and with a heated iron from a hobby store (a regular iron works fine) I touch the iron to the corners of the panel activating the glue. Now with the plastic sheet and glue is attached to the panel so it won't slide around. It's ready to for the dry press.
Dry Press Preparation
I use an old Commercial 200 Dry Mounting and Laminating Press made by Seal Inc. There are many kinds and models. I bought mine from Craigs list for 200$. It works ok but the temperature is a bit tricky to adjust since the knob with the pointer is missing.
To tune up the temperature control I use a kitchen oven thermometer... the kind that you stick in the turkey and it reads out on a digital display on the counter.
CRITICAL NOTE: Most ACM Panels have a max temperature limit.... above which the panel will warp. I try to keep the panel I'm using to a max of 160-165F.
Precautions: (1) Be aware of the temp limits the product you are using, (2) temperature test the heated press you are using for accuracy and (3) make several "dry runs" on a scrap piece of panel.
The bottom plate on my press is about 1/2 inch of felt and the top plate is metal so when I put the panel in I lay the ACM panel on the felt and then put a couple of sheets of craft paper on top of the panel.
To temperature test your heated dry mount unit put the oven temperature probe into the press, close the lid and adjust the temp knob so that the probe reads 160 to 165F. Wait at least 10-15 min for the system temp to stabilize so that you're confident it won't go above 165. The panel will warp if temps go much higher and the glue activates between 150 and 155F, so temperature accuracy is critical.
Once the Press is warmed up and temperature is controlled you are ready to proceed.
Just a note of caution... Don't believe the temperature read out on the press. Test it with the digital kitchen thermometer
Attaching the Adhesive to the Panel
First we attach the glue to the panel. The clear plastic sheet and glue "tacked" to the panel was prepped previously and is ready to go. So we open the press and insert the panel and place a couple of sheets of craft paper over the panel. This keeps the upper platten clean and free of glue.
I leave the panel in for about 60 to 90 seconds. Just enough to activate the glue so it will stick to the panel.
After removing the panel and while it's still warm I use a small brayer to roll over the plastic pressing the glue into the panel. Then I let it all cool for a few minutes.
After cooling the plastic can be removed leaving the glue attached to the panel.
Bonding the Canvas
Carefully lay the canvas over the panel making sure it is square and straight. Note the canvas should be cut so that it overlaps the edges about 1/4 to 1/2 inch. Carefully insert the panel back into the press, once again covering the panel with craft paper.
This time let the panel "cook" for about 4 or 5 minutes. Then remove and roll the canvas down with the brayer making sure the canvas is down securly and there are no bubbles.
Trim the panel with a razor blade and you are finished.