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Repainting my shed (wood siding/trim), the prior owners left behind old paint. The paint they left was oil base, so I went out and bought oil base paint. But I'm concerned that they possibly repainted using latex paint. I read that you can determine the paint type by rubbing the old paint with a Q-tip dipped in alcohol, if paint come off it means you have latex. I did that and the paint came off.

So here are my questions:

  1. Is there a better way to determine if my shed has oil base or latex?

  2. If my shed has latex can I use a water base primer over the latex and then top coat it with the oil base paint I bought?

  3. Am I better off just buying new latex paint?

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Alkyd over latex is touch-and-go - well, it works or it doesn't. That depends on several things.

First, is the latex paint new? If it's less than a year old, forget it - it will not be happy being overcoated with an alkyd (oil based) paint.

Second, you can do an adhesion test. If it passes the adhesion test, you are all set - you can use that paint. Google's not much help here because everybody wants to sell you elaborate paint-test equipment. You only need a razor blade, packing tape, and a cotton ball. And this procedure explains it. The test is fairly over-wrought, but you can shortcut down to what matters to you. I've bolded the parts that I'd do.

  • Find an area which is concealed and won't show too badly, but is intact.
  • Make a cross-hatch of razor cuts through the first few coats of paint.
  • Clean it carefully. Don't leave any soap residue.
  • Do a tape test: stick tape firmly to the crosshatch area, and pull it off, folding the tape back nearly 180 as you pull it off (not straight out). These first few steps are documented in this video. Also it's better to use packing tape.
  • Tape a cotton-ball to the cross-hatched area, and wet it in the paint's reducer (i.e. paint thinner). Use very long runs of tape, as the thinner may try to dissolve the tape goo.
  • Wait 30 minutes, remove it and see if the paint has softened or dissolved.
  • Wait til it dries.
  • Do the tape yank test again.
  • Sand, wipedown and prep the crosshatch area same as you plan to prep the house.
  • Paint it with that paint.
  • Wait til it dries. Look for problems.
  • Do the tape yank test again.
  • Thanks! Were you said "Paint it with that paint. Wait til it dries. Look for problems." How much time needs to go by to make sure it is free of problems. For instance will I know as soon as it drys, or could a problem arise in a couple of weeks? – CRP Apr 1 '16 at 19:52
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    This is , sorry..., ridiculous. You can use modern acrylic/latex (water-based) paints over well sanded and cleaned alkyd resin (oil-based) paints. You can also use modern alkyd resin (oil-based) paints over well sanded and cleaned acrylic/latex paints. Even in poor conditions, acrylic/latex paints cure completely in 30 days, you do not have to wait a year to paint over it. You do not need this overly complicated adhesion test, just sand it well, starting with coarse and working to fine. Scrape any paint that is poorly adhered and make sure the surface is clean prior to painting. – Jimmy Fix-it Apr 1 '16 at 20:22
  • @CRP normal drying time will do, plus a bit of margin. Be aware alkyds can take a day or more to dry, that's what Japan Dryer is for. – Harper Apr 1 '16 at 22:08
  • @JimmyFix-it yes, latex over well cured alkyds with good prep. However that still depends on good prep, which as a rule, most people aren't very good at. Alkyds over latex have been a great deal more troublesome for me than perhaps for you. At one point paint was curdling under my brush, til I asked the facility manager what the old paint was... This was on machinery you would not expect to be painted with latex. It had failed early. Good prep did not make a difference. The test is the industry gold standard and manufacturers will stand behind it. – Harper Apr 1 '16 at 22:18
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Actually a rag with acetone would be better for testing old paint... it would wipe latex off but not oil based paint.

Added: If it wiped off with alcohol... then yeah, you'll probably end up getting more latex. Also, be aware that they might have covered up lead based paint.

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I would say with very high probability that you have latex after the swab test - 98%+.

Easy way to test latex paint... boiling water. If you hit the same area with boiling water a few times the latex paint will get soft. It is pretty obvious and usually you can do this without permanent damage - stop as soon as you see it getting soft and retaining water.

  • on painting you just have to read the instructions. Almost all oil based primers are meant to go over latex.

  • I would use an oil base for anything exposed like a shed. For sure if it is metal.

  • repainting your shed, if you don't take off old paint you are reliant on that paints current bond for the top layer. You are hoping that the top layer completely protects the bottom layer. I am not telling you to scrape off old paint because sometimes that just isn't worth it but I am saying you might want to be generous with the amount of paint you use.

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You could wash the shed down and allow it to dry completely, then apply an oil-base primer to all surfaces. Allow sufficient amount of time for that primer coat to dry then you can use two coats of latex paint to finish. Testing the existing coating won't be necessary if you use this method. Removing dirt and grime from surface is key.

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