Polystyrene Trim Finishing Tips

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When dealing with finished trim you'll need to do some finishing work, no matter how good your carpentry skills. Painted trim is easy to finish using caulk and paint, but clear hardwood trim and polystyrene trim are more difficult to finish. In this article we'll take a look at finishing pre-finished polystyrene trim. Our Polystyrene Pros and Cons article covers basic info about polystyrene trim.

How to finish polystyrene trim

Finishing trim work usually involves filling nail holes and any small gaps in the miter cuts with caulk or wood putty, then painting the filler. The pre-finished veneer on the polystyrene trim is very thin and when sanded, it scratches easily. The first time we installed polystyrene trim we used wood putty to fill the nail holes, and even though we used the recommended 220 grit sandpaper to smooth the excess putty, the veneer showed some minor sanding marks.

Filling nail holes in polystyrene trim

The second time we installed the trim, we used DAP Dyna-Flex vinyl spackle to fill the nail holes and gaps. The wet spackle was wiped down with a moist, clean rag so no sanding was needed, and the spackle was allowed to dry. We have a Lowes and Home Depot nearby and took a piece of the polystyrene trim to Lowes to try a color match. The different colors in the woodgrain finish can make color-matching difficult, but it matched fairly well. The image below shows the miter gap on the right, and the miter gap filled with vinyl spackle on the left. *Note - We're normally much better trim carpenters than that - the large gap was intentional for demonstrative purposes.


Polystyrene corner trim nailed to wall

The other issue is applying the paint. Because the paint probably won't match perfectly, you'll need a really thin paintbrush so the paint stays on the spackle, not the trim. The image below shows the tub of spackle on the left, and the trim with a first coat of paint on the right. The two nail holes on the left of the miter cut need another coat, but the two on the bottom of the miter cut are almost invisible. The miter cut is wider, so it took several thin coats of paint.


polystyrene trim and vinyl spackle

Buying trim finishing supplies online

If you don't have a Lowes or Home Depot nearby for color-matching the trim, Amazon carries MinWax finishing markers (ad) and blending pencils, but choosing the right color markers might be a challenge. Amazon also sells a version of the DAP vinyl spackle, but it's called AlexFlex, not Dyna-Flex. It looks the same and isn't expensive if you buy the small tub.