MITRA Forum Question Details

Hello there,

Generally speaking I'd be a little afraid of painting over plastics because if paints stick to it, they generally form a weaker bond than on other mineral/plant based surfaces, and they don't have the mechanical impregnation through the porous fibres since the polymer likely isn't.

Since you have less van-der-waals adhesion and no mechanical anchor for your paint, it's easier for your paint, whether it's acrylic or oil, to peel or flake out at some point.

Sizing won't be necessary as it means filling the fibres to protect them, giving them elasticity and providing a tooth for the next gesso layer.

The polyester is unlikely to rot, may not have the elasticity you'd appreciate with cloth sized with rabbit skin glue, and won't have the easiest adherence for the next layer. 

So sizing indeed is not required per se. A gesso  or ground first layer that completely wraps the fibres, going entirely around them thoroughly, I think would be a good thing to insure your paint won't flake afterward. (Give it an achor around the fibres instead of through them)

Alkyds generally have a better adhesion on plastics than acrylics, so I'd chose that. Oils directly I'm not sure, I don't know the differences in surface tension between them. I would still bet on alkyds though.

​If you want to do a nightmare test, just knife apply a bit of thick acrylic on your polyester, and try to peel it off once properly dry. Worth testing against thorough gesso or alkyd.

Good and safe paint to you.

Lussh

​I've used a roll of polyester sold at an arts supply.  And found for this particular canvas (other may vary) that it was best when sized first with an acrylic medium, then with two or three coats of acylic gesso finally with an oil ground on top of this.  Not to remove oil contact, but to stiffen the canvas which I judged it definately required.  This polyester was quite thirsty with the acrylic medium, but stiffened it up to a very high degree while still being flexible enough to roll.  One drawback I though was strike through to the stretcher.  So to solve this I would do this sizing pinned within a frame and after drying, tack around a stretcher to prime.  This thirstyness I mentioned put me off just using a alkyd ground.  As in a test, it soaked up so much of the vehicle that I feared it might now be underbound.  Though it appeared to have no adhesion problem.  Also it occured to one that this ground might increase in brittleness with time and the only support stiffness in this test sample was coming from the paint itself.

Marc.​