Painting walls and ceilings nothing speical

[HawkEye244]

Hi all,
Other than wash down walls and ceilings with a bit of sugar soap and then water prior to painting, do plastered surfaces benefit from a rough sand to key them ? Why is keying needed on wood but not plaster? Thanks

 

[opps]

I sand new plaster as a matter of course but that is to remove any imperfections. From time to tie you do come across plaster than has been "polished" too much by the plasterer. Painting over it is like painting on glass and sanding definitely helps.

Standard matt emulsion on plaster is quite porous and accordingly additional coats of matt emulsion don't require a key.

I must admit that i have never stopped to think about non-porous finishes such as waterbased eggshell on walls. I guess that they would benefit from a key when applied over existing wb eggshell.

I have seen WB eggshell applied straight over OB eggshell without a key, it didn't pass the "fingernail test".

 

[HawkEye244]

I sand new plaster as a matter of course but that is to remove any imperfections. From time to tie you do come across plaster than has been "polished" too much by the plasterer. Painting over it is like painting on glass and sanding definitely helps.

Standard matt emulsion on plaster is quite porous and accordingly additional coats of matt emulsion don't require a key.

I must admit that i have never stopped to think about non-porous finishes such as waterbased eggshell on walls. I guess that they would benefit from a key when applied over existing wb eggshell.

I have seen WB eggshell applied straight over OB eggshell without a key, it didn't pass the "fingernail test".

Good info, thanks bud!
Want as good a finish as possible without giving myself work.

 

[sparkwright]

Reading that reply suggests new plaster, but reading the original post it suggests it's already painted plaster.

I read it as painted plaster.

In which case give it a quick rub down all over. Don't go too crazy. It will help if there is a problem. Usually emulsion can go straight over matt emulsion, but a quick rubdown will get rid of any bits in the old paint.

Be sure to thoroughly remove any sugar soap, if you don't it may react with the paint.

Rubbing down wood is more important usually, as new paint doesn't stick very well to glossy surfaces.