Paint Additives for Glass-Smooth Brush Finish

      Advice on achieving near-perfect flow-out and smoothness with roller-applied and brushed-out paint. May 7, 2009

Iím using 100% Benjamin Moore "regal" satin/pearl finish waterbased paint. My goal is to apply on Gatorfoam (like foam core board except it is laminated with a wood type product and has the surface quality of craft paper) with a brush to get a glass like finish as if it was sprayed. The material and room are 60-65 degrees and the relative humidity is 20%.

With multiple tests the best finish has been to apply paint with a 1/2" nap roller to get some build then strike off/level surface with 3" nylon professional brush. Material thinned 30% (15% distilled H20 and 15% Floetrol). I read that the PG (Propelyne Glycol) should first be mixed 1:1 with H20 then added to paint. I want the surface to level but I need it to cure quickly. With the above 15/15% it can be lightly sanded within two hours at the above conditions. I have gone up to 20% on the Floetrol and no H20 and the surface is better with the H20/Floetrol mix. I also tried a foam brush for strike-off but seem to get better results with the nylon brush. Does anyone have any recommendations on proper % thinning on PG or a combination of Floetrol and water?

Forum Responses
(Finishing Forum)
From contributor G:
You are asking a question that only trial and error can answer. You might consider surfacing the gatorfoam first to provide a smoother substrate. Extending the dry time will allow better leveling. Do you need to sand to get a smoother surface? Why do you not just spray your paint?

From the original questioner:
In an office environment habituated by people does not give me the option to spray. The surface prep is very smooth. I knock off any imperfections with p600. The trial and error method can be a lot of guessing and time consuming. I'm looking for someone with experience using PG and what a good thinning approach (% by volume) would be. This may not be the right approach, that's why I am asking.

From contributor D:
There is no true answer for your question; how much thinning... proportions: The answer will be dependent on too many factors. Trial and error are the only way to find your ratios! I have found that adding some H2O reduced poly to Ben. Moore paint and will allow better flowout, with H2o and PG added. It also adds some strength and hardness. I know it is against traditional wisdom but it works.

I also work in occupied spaces. I ask to be allowed to work in the off-hours. For office space I usually ask to work from 6:00 pm to 2:00 am this affords me the ability to mask off and spray. It also allows the finish to dry before the space becomes occupied again. Yes, I do charge extra for this service, but the job moves faster, there are less complaints from workers (mine and office staff) and the finished results are better.

From the original questioner:
Is the PG typically mixed 1:1 then added to the paint? Is it realistic without brain damage to get rid of the brush marks on a hand applied surface?

From contributor D:
1:1:1 (PG:H2O:poly) then added to the paint is a good start. You will need to adjust from there. The adjustments needed will be determined by trial and error. If the paint dries too quickly, add more PG and water. If itís too runny, use less water and so on. The ratios will be different on every job. Sometimes they can be different from day to day on the same job.

Brush marks are a real problem, especially with a latex finish. today even oil based (alkyd) paints don't work as well as they did, without adding product to replace some of the VOC's that have been removed to make them more "green" and "legal" to sell. I would probably opt for a foam roller or no more than a 1/4" roller to apply the paint. 1/2" will impart too much stipple. I have even used handled painting pads (3x6?) with good results on flat surfaces. Practice on your sample boards and good luck.

From contributor V:
I agree with contributor D on the foam rollers. They are quick and very smooth - a tip off right away. The Purdy brushes are decent but even better are water color wash brushes (finer softer bristles). I usually just use water for thinning. Roll on with the weeny foam and tip off with the wash brush. I like to scuff sand with 180 grit sponge before the final coat. That gets it pretty darn smooth. If I need smoother than that I buff and polish.

From the original questioner:
Do you get a wash brush from an art store? Do they make them 2-3" in width?

From contributor V:
They sure do. They arenít cheap but still inexpensive compared to spray equipment.

From the original questioner:
For the $50 compared to the Purdy for striking off latex is this a marked improvement for brush marks?

From contributor V:
It will make your Purdy look like a graining brush. I like Purdy brushes, but when I need smooth they are not what I use. I usually use a two inch as a light hand is important for tipping off. Three inch might be even better but could also be less good.

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