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I am curious when a surface should be reprimed (if ever) vs just repainted. Are there any general guidelines? If it is badly stained, should I reprime or use some stain blocker?

Please consider this picture of a 30 year old casement window (wood). I stripped some paint from the trim around it and the stripper got on the window + just regular grime settlement over the years. I would like to repaint white semigloss but am not sure if I need to reprime first to block the ugly stains.

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Primer is generally used for one or more of these reasons. If they apply, use it. If you're unsure, use it.

  • Save cost (cheap sealing and color/texture uniformity coat)
  • Improve adhesion over glossy or dirty surfaces
  • Block stains
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  • Do you think in the case of my stains I should use it? – amphibient May 25 at 20:48
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    Seems to me all three reasons apply. – isherwood May 25 at 20:59
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If you try to lay a topcoat over an inconsistent surface, the inconsistencies will "print through" and give an inconsistent result.

Look at what you have there, you have a dog's breakfast of colors and surfaces. Different colors, stripper-etched vs normal, stains, and whatever that caulking or filling material is. (you're in trouble if it's silicone).

So if you just merrily slap topcoat on this, all the differences in texture, color and gloss will "print through" the new topcoat.

That's what primer does for you. It goes over everything and provides a consistent surface (if you paint enough coats). So when you topcoat, you get a consistent result.

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  • "you're in trouble if it's silicone" - not necessarily. It's fairly trivial to prep silicone so that it accepts paint. – MonkeyZeus May 26 at 15:22

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