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Preparing Mild Steel as milled for Painting  

June 14, 2008

I'm a small business owner in metal fabrication (primarily "mild/carbon steel"). I do a variety of projects from custom made roof racks to wrought gates and fences. I've switched to a water-based acrylic paint because of the ease (and environmentally friendly) clean-up. However, I've had a very difficult time getting consistent adhesion. Sometimes the paint will peel off like dead skin after a sunburn. I've tried detergents and degreasers, in a very labor intensive scrubbing manner, without consistent results. I read another poster's question (letter 1264 from Art Thompson) about Muriatic Acid [link is to product info at Amazon]. I have heard of acid cleaning but maybe mistaken about the term "acid bath".

1) Do the parts need to be submerged for some period of time or can they be sprayed and allowed to sit? Most of the projects I build are too big to dip or submerge. Even the smallest/shortest part may be 10 feet long). Can muriatic acid be simply sprayed on the project? If so, how long should it be allowed to stand before rinse and paint? If not, is there a non-labor intensive process whereby the as-milled steel can be treated prior to painting?

2) I've gathered that muriatic acid will take care of the scale on hot rolled steel. Does it also take care of the oily film on cold rolled parts or other "pickeled and oiled" steel (i.e. expanded sheet steel)? Many of my builds include a combination of hot-rolled and cold-rolled steel. Is it possible to have a single solution for both?

I'm a small business owner and don't have the facilities to create a 10,000 gallon dipping station. I would appreciate any help you could offer. Thanks.

Sincerely,

John Faulkenberry
Metal Fabricator - Suwanee, GA, U.S.A.

Waterbase acrylic pefers a surface that is fairly clean and warm prior to being painted so to maximize your adhesion without using acid. As you probably know the EPA doesn't approve of pretreatment operations that involves contaminated "runoff", so my advice would be to wipe down your pieces with a mixture of 2/3 water, dawn detergent and 1/3 EB (butyl cellosolve and get the steel to 120-130F prior to painting.

Nick Nickelsen
- OKC, Oklahoma

You might be able to blast it with a 250 to 350 aluminum oxide and get a really clean surface that will hold paint. Sound to me that you have a thin mill scale that is thicker on some parts than on others.

James Watts
- Navarre, Florida