How to paint over exterior oil paint?
August 25, 2017 10:07 AM   Subscribe

10 years ago I built a wooden shed and painted it with oil paint. Now it needs repainting and no one sells oil paint anymore. What to do?

When I built the shed, all the advice I got was to use oil paint on it as it will last longer. I should have listened to the one sales guy who said he thought it was likely that oil paints were going to be banned in the near future.

The current paint is in great shape on the two sides of the shed that don't get any sun but completely blasted, faded, chalky, and peeling on the two sunny sides. It isn't a very big shed.... only 10'x10'. The exterior is plywood with battens (which will make sanding and scraping it down more annoying). We were given a quote by a painter of $1200 for the job ($200 for paint, $1000 for labour), which is way more than I can justify.

So, how do I paint this thing? Does anyone have any successful experience painting over oil with a water-based paint? Advice on the intertubes is conflicting - scrape and sand, use a de-glosser, don't use a de-glosser, use an alkyd primer, use a water-based primer. Lots of horror stories about the new paint just falling off.

I just want the thing done, hopefully with a minimum amount of sanding, and I want the new surface to stick.
posted by fimbulvetr to Home & Garden (6 answers total)
 
We just bought Benjamin Moore exterior (aura) for the top coat (acrylic) and sherwin williams fast-dry oil based exterior primer to re-paint our house. We were painting over oil-based paint. So far, seems to be working just fine.

The usual "wash well, scrape and patch before painting" applies.
posted by k5.user at 10:14 AM on August 25, 2017


It's been mumble years since I worked in the paint department of a hardware store, but the advice at the time was that latex over oil is fine (wash, sand, patch, prime, definitely) but never the reverse - something about oil needing to "sink in" but latex "sits on top", so oil will never stick to latex (nothing to sink into) but latex is fine on oil (lays on top).
posted by okayokayigive at 10:51 AM on August 25, 2017


I would scrape away loose paint. Give it a light sanding all over with 150-220 grit. Fold the sandpaper sheets in half, and then in thirds. Even with the battens, it won't be that bad. You just want to take the gloss off. Then cover it with KILLZ primer. Finish with latex of your choice.
posted by humboldt32 at 12:10 PM on August 25, 2017 [1 favorite]


Ive heard latex-over-oil and never oil-over-latex because latex has more swell-shrink in temperature swings, and that's tolerable as a top layer ( latex remains stretchy) but delaminates as a middle layer.

Anyway, I third the advice to scrape fiercely, then sand some, then wash, then let dry very well, then paint over with whatever a good hardware store near you advises (local advice useful because the worst weather varies so much from place to place; hardware store rather than paint store because I don't think you want the Newest Thing, we don't know if it lasts yet).

White or very pale paint is probably more durable than dark. Also, be conservative about the temperature and humidity and recoat time listed on your paint.
posted by clew at 1:22 PM on August 25, 2017


Alkyd paints are gycol based paints, a derivative of soy oil rather than linseed or safflower oil. They dry faster, stronger and more uniformly than "enamel" type house paint. Alkyd paints should be fine for applying over a dry enamel surface.

You can avoid the need for solvents by adjusting the viscosity at the paint store, careful application, and using brush soap or discarding the brush at the end of your painting.

Many paints for household use are formulated to shed titanium pigment in order to maintain the white appearance of an exterior, which is most likely why some of the surface is "chalky".

The paint layer is likely cracking from sun exposure, and delaminating from the plywood. The only way to prevent that from recurring is to shield the surface from repeated sun exposure. Delaminating from the substrate will not be remedied by applying further top coats to the failing underlayer. For a smooth surface, sanding down the cracked layer would be the best practice.

A Makita palm sander would make it easier for you to sand the batting and reapply a fresh top coat.
posted by effluvia at 3:53 PM on August 25, 2017


You can do this the quick and easy way or the hard and more correct way. The easy way is what everybody used to do before power tools, which is take a scraper to it, knock everything that is loose off, and maybe do a rough sanding and damp wipe to remove dust, then apply paint (over primer if you aren't lazy). Where the existing paint is still well adhered, a second coat should stick fine. It'll look a bit ugly because the texture of the remaining old paint will show through. I actually kinda like it for outbuildings, TBH. Gives them a bit of an "old" feeling.

The right way to do it is to scrape until you get everything you reasonably can get and then sand it really well to knock down all the edges of what little paint you leave behind, then wipe, then prime it with a primer known to stick to oil and that works with latex. (Kilz is probably a good option), then painting over that after another light sanding/wipe down.

It'll look better, but I seriously doubt it will last any longer than doing it the slightly easier way.
posted by wierdo at 6:11 PM on August 26, 2017 [1 favorite]


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