When to use a primer is a question that comes up a lot when discussing interior painting with DIY'ers.
Primers are formulated to do functions that finish paints don't do very well. There are "universal primers" and there are specialty primers. For the most part primers are used for the following reasons:
1. To ensure adhesion to the underlying surface
First and foremost primers are used to tie the finish paint to a surface that it normally doesn't adhere to very well.
Examples: Painting over glossy surfaces ( glossy paints or plastics etc...)
Painting over smooth, hard surfaces (ceramic tile, metal, oil paints etc...)
Primers manufactured to promote adhesion are often referred to as "bonders". Usually bonding primers have good adhesive properties to a wide range of surfaces. Of the bonding primers the alkyd and shellac primers are probably the best, the waterborne primers often need to cure before they develop adhesion which can take a bit of time - usually about 30 days, which leaves them soft and easy to scratch off until they finally cure.
A good alkyd bonding primer is XIM 400 or Zinsser Coverstain and a good shellac primer is Zinsser"s BIN primer. These primers have good adhesion to a wide variety of hard to stick to surfaces.
b. Painting with deep tone finish paints
The problem with coverage and hide when using deep tone colors is that they are mixed from a clear base. The "hide" in paint comes from the titanium dioxide pigment used in other bases, which is a white pigment. With deep colors you cannot bleach them out by tinting them in a white base - so they are tinted in a clear base which is somewhat transparent. In order to help hiding with deep tones you will need to use a primer that is darker to block out the underlying surface and help the finish paint cover. The way this is done today is by using various shades of gray for the primer coat. Borrowed from the automotive painting industry, paint manufactures of house paint have found this works best with deep tones.
Sherwin Williams has a system of six different gray primer colors ( P1 - P6 ) gauged and cross referenced to each of their deep tone finish paints, all you need to do is put on the primer they have referenced for the particular deep tone paint you are using.
Can you paint a latex paint over an oil primer?
Absolutely. Oil primers that are pigmented (not clears), are intended to be recoated. Suitable paints for recoat are latex or oil.
Can you use a latex primer over an oil based paint?
If the oil is glossy, use only a latex bonding primer. Xim UMA is a good one. Be sure the oil painted surface is clean - this is very important.
2. To promote hiding and paint coverage
If you are painting over a dark wall or if you are going to be using a deep tone finish paint color, you should prime before painting.
a. Painting over a dark surface:
In this case, as long as the underlying surface is not glossy or needing a primer for other reasons, you don't have to prime but priming with a PVA type drywall primer will save you money since you will be doing two (or sometimes three) coats, do the undercoats in a inexpensive PVA primer tinted to match the finish paint.
Bonding Primers: Kilz primer, BIN, Coverstain are a few.
Interior Paint Primers:
When Do You Need To Use Them?
part 1
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