De-glossing paint for repainting


This question has come up a couple times recently in my work. I have some clients who need stuff repainted that's currently painted with semigloss paint (latex). I know you need to "scuff up" the surface in order to get new paint to stick to it.
My question is, is it better to physically scuff the paint with sandpaper, or to chemically etch it (someone mentioned using TSP in another thread)? I suppose it may depend on the specific situation, right?
One thing I do know about TSP is that if you use it, you'd better make damn sure that you remove all of it before you paint, else the paint won't stick worth a damn.
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David Nebenzahl wrote:

paint and I use semigloss both places.
I always wash with TSP and have never had paint fail to stick. The key may be too use a lower concentration of TSP than recommended. My box say 1/2 cup per gallon but my concentration is one-forth to one half of that. Second, I rinse it with with fresh water. I use a wash rag and ring out most of the water, wash or rinse, and rinse the rag with every few swipes when rinsing.
Of course, many professional painters now use the silicate substitute for TSP because they say you don't need to be as careful when rinsing.
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