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The outside window sill of my basement window has weathered considerably, with missing paint.

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So, the good news is, that I have some original paint left (house was built 10yrs ago) that I could use which saves me the trouble of matching the color.

The issue is that when I read the labels, the instructions are worrisome:

  • Industrial use only.
  • Petroleum based.
  • Can cause organ damage.
  • Can cause cancer.

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From the label: naphta solvent. 1,3,5-Trimethylbenzene.

I am hesitant to use it.

I don't mind buying new paint, but: can I put water-based paint on top of wood that was previously painted with chemicals from hell? Or should I use the can I have?

I am in Canada, by the way. It would not suprise me that stores would only carry water-based paints now.

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Provided you do correct surface preparation, you should be able to prime and paint over top of the alkyd base without trouble, given its age being over a year.

Don't be gripped by fear

But your hysteria over chemicals is misguided. Billions of people use paint responsibly and don't die as a result. In fact, cans of "spray paint" are the exact same stuff. Except you are also huffing the resin and pigments, since those are being atomized into the air.

Fact is, humans who aren't formally educated in risk assessment are absolutely terrible at risk assessment. They drive without a care to the airport then need to take a Xanax to set foot on a jetliner operating under FAA Part 121. That's not even wrong SMH. They go "OMG that's plutonium! I won't touch it!" to darn near everything that they hear about, in other words, they react emotionally to media and memes... but blithely ignore many other things which can actually give them cancer or a gruesome disease.

If you doubt that, you only need look at people's reactions to obesity and COVID-19 - who is genuinely scientific about their risks to those? I rest my case!

I paint alkyds all the time, and I paint much harsher stuff (the 2-part LPU paints which will damage your immune system fast if you huff up the overspray). If anything the alkyd may be safer for you - you'll be motivated to use the best PPE, whereas with emulsion/latex paints you might be cavalier.

But I wouldn't recommend alkyd paints anyway

That said, I wouldn't wish alkyd paints on anyone who won't embrace the trade-offs to get the higher performance. It requires different brushes (cleaning brushes is hard and very messy so I just use "chip brushes" and toss them), it's stinky, it's very slow to dry so you must mind timing and weather much more carefully, it emits stink for days, and cleanup is a nightmare compared to emulsion/latex. All that adds up to "it's not for you" and that's fine on its own merits.

Please - dispose of properly.

Dispose of the paint properly, either via household hazardous waste or giving it to someone who will use it. An old friend says "the best way to dispose of old paint is paint something you don't want, then throw it away" - but I gather that's not for you. That's environmentally valid - it assures the paint cures properly into nontoxic products, instead of leaching uncured component chemicals into the environment. I myself would paint a few "drift cards" so you have a reference for the color contained therein... however, it doesn't really matter.

Matching doesn't really work

So, the good news is, that I have some original paint left (house was built 10yrs ago) that I could use which saves me the trouble of matching the color.

That's not going to work. Over 10 years the paint color and lustre will have changed somewhat from weathering. At least with drift cards you can get an exact match for what was intended. There's no good way to copy-paste a color, because there are simply too many possible colors for paint store chip cards to be useful.

Also, the lustre of any emulsion/latex paint will be "not as good" - high gloss latex isn't really feasible.

I am in Canada, by the way. It would not suprise me that stores would only carry water-based paints now.

No graffiti in Canada, then? Like I say, spray paint is this exact stuff. And I guarantee I can walk into any better paint store and get alkyds custom mixed.

In America they want to know my application (to know if it's exempt; if you use alkyds for house paint you must buy it in quarts). Coincidentally when I tell them my unusual application, the store is often mysteriously out of gallon cans :) I don't think they believe me :) I guess people paint houses OSHA Orange?

Heck, you can use this stuff in L.A. if you buy it in quarts. Well, litres.

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Assuming the paint is still good. I would use it. Use proper precautions, use nitrile gloves and an organic respirator. If it's no longer usable dispose of properly and buy new paint. I wouldn't worry about the paint that is already on there. The existing paint doesn't appear to be peeling and with minimal preparation, removing any loose paint and a good primer you should be good to go.

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I can't really speak to what you should do on an ethical or environmental level. Your call. But the fact that the paint is at least 10 years old suggests that you'll want to buy new paint. It might still be good, but paint has a finite shelf-life.

You can use water-based paint over cured oil-based paint if you do a couple things first. Water-based paints sometimes don't adhere very well to oil-based paint in part because of the smoothness of the finish the oil paint is likely to have. You might want to scuff up the surface of the existing paint by lightly sanding with something like a 220 grit sandpaper, but since it's outside the glossiness might not be a problem.

Wash it down with TSP (gloves, goggles, etc. should be utilized.) Apply a coat or two of bonding primer so that the old color can't be easily seen through it, and then a coat or two of your new paint color.

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