overpainting a wall

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Hi, sorry if I am posting in the wrong place.

I am trying to help my brother get the right paint for his walls. He tested them by putting methylated spirits on a cloth then rubbing it on the wall. No paint came off, I believe this means the paint on his walls is oil/solvent based.

Should a solvent based undercoat be used? Is this the same as a solvent based primer or is this a separate item? If he does indeed use a solvent based undercoat, which type of top coat does he use and is the top coat the colour you want to put on the walls?

Many thanks
 
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Solvent based undercoats and primers are different products. The latter is used to seal porous surfaces (eg timber). Undercoat is used to prevent gloss paints from sliding (and to provide better adhesion).

If the walls are currently oil based eggshell or dead flat oil based paint, sanding them with 120 grit paper will probably be sufficient to provide a "key" prior to using a "full fat" waterbased emulsion.

Whilst it is true that alcohol will soften waterbased emulsion, simply wiping a rag dipped in meths over the wall should not remove anything. I'd suggest letting the kettle boil away next to the wall and then wiping the condensate form the steam away. If the paint wipes of it is water based emulsion. If the paint becomes soft it may be waterbased eggshell.

As a professional decorator, I have used oil based paints on walls, but they are seldom used these days (it takes much longer to dry and stinks for days, and costs more). Although far more durable they are prone to discolouring/yellowing (especially behind furniture/fittings/pictures where the UV sun light light cannot "see").

In short, sand the wall to aid adhesion and then apply two (or more coats) of the coloured emulsion desired.
 
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