Scene 1
A rusted control arm or coil spring or push rod rests on a table in your garage. The part's been there for months waiting to be restored. You intend to sandblast it and paint it with two coats of POR-15^® Clear Rust Preventive Paint. You want the look of bare, clean metal through the glossy coating. You are an experienced restorer.

Scene 2
Your buddy has sandblasting equipment. You blast your control arm or springs or whatever, wrap it in newspaper or brown kraft paper and take it back to your garage, making sure you don't touch it with your bare hands. The weather is sunny, or overcast, or rainy, or you choose it.

You open your can of POR-15 Clear and lay down a thin, covering coat. The part looks gorgeous. Four hours later, or the next morning, you put down your second coat. Looks great.

Scene 3
A week later or two weeks later you examine the part and see a rust-colored spot or two under the coating, which still looks otherwise perfect. What's going on here? This stuff is supposed to stop rust. Have I done something wrong?

Here's What Happened
Now let's go back to the beginning where the rusty control arm is resting on your bench. It's been indoors for months, so it must be dry, right? WRONG!! Ordinary steel is very porous. That control arm has thousands or millions of microscopic cavities in it that are filled with moisture. Even after sandblasting, thousands of moisture pockets remain. The piece may look dry, but it isn't... and if you let it sit in your garage after blasting for a few hours before you paint it or worse, overnight, more moisture settles in to haunt you later. POR-15 is hydrophilic (moisture-seeking). The first coat 'pulled' that residual moisture to the surface and sealed it in, as did the second coat. Microscopic corrosion then occurred under the coating and became visible to the naked eye. The POR-15 coating did not break because it's flexible and very tough, but if enough moisture remains under the coating and the corrosion continues for a while, that may occur in the future.

Get The Moisture Out First
After you've prepared your part for painting (by sandblasting, metal-prepping. etc.), use a warm air blower to make sure the piece is bone-dry, then paint it at once. Apply your second coat or POR-15 when the first coat is just dry to the touch with slight finger-drag remaining. And never forget the three most important rules of painting: