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shellie.elyse 10-07-2008 12:06 PM

Painting over high gloss
 
Describe the steps for painting over a high gloss paint.

Matthewt1970 10-07-2008 01:44 PM

Are you sure it is High Gloss and not a semi-gloss? A lot of people have made that mistake and it is an easy mistake to make.

Odds are if it is truly a High Gloss then it's oil. Yes there is high gloss latex, but high gloss is just soo much better in oil. If it is Oil, then you can just go right over it with oil again. If you are planning on going over high gloss oil with a latex, I would recommend priming it first with an oil primer. Then the latex will stick better. If it still has a lot of shine to it, you may want to sand it to give it some grooves for the primer or paint to bite into.

sirwired 10-07-2008 01:59 PM

The basic way to topcoat glossy paint is to lightly sand the existing paint, remove the dust, prime with a bonding primer (there are some excellent water-base bonding primers that work on top of oil) and then topcoat with the quality paint of your choice. Personally, I prime with SW PrepRite Pro Block Latex, but there are others. My topcoat of choice is SW ProClassic Waterbourne Gloss.

SirWired

Nestor_Kelebay 10-07-2008 10:27 PM

The first two questions I would ask are: Is it a latex paint or an oil based paint? And, if it's an oil based paint, how old is this paint?

Clean the paint with nail polish remover and if it dissolves the paint right away, then it's a latex paint. If it takes some time to soften and finally dissolve the paint, then it's a oil based paint.

If it's an oil based paint, it wasn't until the 1980's that linseed oil based paints were largely replaced with alkyd paints. If the paint dates from 1980 or before, my guess would be that it's a drying oil, in which case you can simply clean it with TSP to dull the gloss.

Animal fats and vegetable oils are all "triglycerides" which means they all consist of three fatty acids connected to a central glycerine molecule. It's the presence of something called "unsaturated sites" on those fatty acids that make some vegetable oils (like linseed and Tung oil) solidify when exposed to air. Alkyd resins are made by stripping those vegetable oil fatty acids off their central glycerine molecule, chemically modifying them to greatly increase the number of those unsaturated sites, and then recombining them with smaller amounts of both glycerine and something called "phthalic anhydride" to make a "clump" of souped up fatty acids. (That's because each of those things will react chemically with the other two, so the result is a "clump" containing mostly souped up fatty acids.) Those clumps are then dissolved in mineral spirits and sold as "alkyd paint".

Because alkyd paint still contains a lot of those vegetable oil fatty acids, TSP does attack alkyd paints, but not nearly to the degree that it attacks and dulls the gloss of linseed oil based paints or Tung oil based varnishes. However, there is a wide diversity in the amount of fatty acids used in making alkyd resins. "Long oil" resins contain up to 75 percent fatty acids, where as "short oil" resins might only contain 50 percent fatty acids. So, if you have some TSP, it's worth it to try using TSP to dull the gloss of an alkyd paint as well.

You can dull the gloss of a high gloss latex paint by cleaning it with a green Scotchbrite scouring pad (of the kind sold in grocery stores for scouring pots). The point here is to roughen the surface so as to increase the surface area the new top coat has to stick to, thereby improving adhesion of the new coat to the old paint. Removing dirt is a secondary concern. So, if you clean with a Scotchbrite pad, either don't use any detergent in your water, or use very little. I think Mr. Clean recommends a dilution ration of 100 parts water per part Mr. Clean, and I think that's probably even too concentrated. The instructions on the ChemSpec Formula 77 carpet shampoo I use say to use 2 ounces of soap concentrate per 5 gallons of water, which is a dilution ration of 3 parts soap to 1000 parts water. I don't think you need to go any more concentrated than that to get optimal cleaning either.


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