Primer for old oil-based paint

Question

I'm confused and hope you can help. I thought ANY primer - I just bought five gallons of Kilz latex primer - could be used over old oil-based paint and then a new coat of latex paint applied successfully. Today, per Home Depot recommendation I bought a gallon of the oil-based Kilz for a couple of small jobs. I thought I'd read that latex primer over oil-based paint is just fine. Please help.

Answer 1

You can overcoat over any old paint if these things are true:

• The paint is fully cured (which can take up to a year or two for single-can paints which rely on air or atmospheric moisture to cure.) Curing is not the same as drying. Curing is the paint molecules "holding hands", polymerizing into much longer molecules.
• You scuff-sand the old paint so the surface has microscopic ridges and valleys ("tooth"), a Scotchbrite pad will suffice, this will remove the gloss from the paint. Be careful not to blow through corners and ridges.
• You clean the surface thoroughly to remove grease, oil, dirt and the like. Especially important in kitchens!

Classically one might use TSP (Tri-Sodium Phospate) to attempt to both clean greases and etch the surface.

Where does primer enter the picture?

• You want a primer over bare material. Original oil Kilz is high-performing over wood. It's not the very best; that's a marine 2-part epoxy primer. Over aluminum or steel you'd want a zinc chromate primer (the classic green color of unpainted aircraft), over steel I just tend to go Rustoleum.
• When you have dissimilar surfaces or colors, and you want to normalize them so they don't "print through".
• If your topcoat doesn't want to stick to the surface as-is, this is called a barrier coat.

If you don't have any of those situations, a primer may not be needed.

I am no fan of the Kilz latex primer; I feel the oil-based product is far more suitable for wood. Perhaps the Kilz latex makes sense over drywall; you certainly do need a primer for that.