Stripping Paint and Rust from Metal

Something that I get asked a lot about is the process for stripping paint and rust from metal parts. Often with metal bits that have been subjected to high humidity atmospheres, such as outdoor bits, vehicle parts, radiators etc. once the paint has been stripped off there is quite a lot of rusty bits that need to be sorted before repainting can take place.
I will deal with getting the paint off first.
Biostrip is really good for stripping paint from metal, and it will strip a wide variety of paint finish's including fusion dried powder coats (even difficult to remove polyesters), tricky stuff like hammerite paints and two pack epoxy's, and modern water based automotive type paints, expecially the notoriously difficult to strip two pack water based auto finish's.
The easiest way to strip the paint is to use Biostrip 42, which is a thin and runny version of biostrip and into which metal parts can be dipped. Normally at temperatures above about 25C a single coating of fusion cured powder epoxy paint will take about 20 minutes to strip. Water based 2 pack automotive paints can strip in as little as 10 minutes depending on the paint type and thickness. Really difficult coatings such as MOD Nato type paints, or things like Caterpillar yellow used on diggers etc can take a bit longer, but usually no more than an hour. A typical domestic radiator with 2 coats of paint, a water based electrocoat primer with a fusion cured powder top coat, normally takes about 20 mintes to strip both coats.
You can tell when the paint has stripped from the parts as it blisters away from the metal surface in nice big sheets which drop away from the part as it is removed from the biostrip, or is easily washed off with water, from say a hose.
Once the part has stripped then it needs rinsing with water to remove any residual stripper which would cause subsequently applied paint to blister if left on, drying and then is ready for repaint. One thing that Biostrip does not do is attack any pre treatment layer that might already be on the metal e.g. a previously applied phosphate coating. This means that parts do no need any further pre treatment other than water rinsing.
If a dip tank is not available, which may be the case if only a small number of parts are to be stripped, then the thick version of Biostrip, called Biostrip 20 should be used. This needs to be simply brushed onto the parts, left until the paint blisters and the blistered paint scraped or washed off. The part then just needs rinsing in water or wiping with a clean wet rag, drying off and re painting.
How to Deal with Rust
If the metal is rusty after stripping then the rust needs sorting before a repaint. There are basically 2 ways to do this:

• Chemically remove the rust using an acidic rust stripper. Acidic rust strippers such as the Biostrip Acid Rust Remover are based on phosphoric acid and will dissolve the rust and replace it with a layer of iron phosphate. The Biostrip 42 Rust Remover is a thin version that can be put into a tank and the part dipped in and left until all the rust has been dissolved. Depending upon the thickness of the rust this can take anything from a few minutes to an hour, and is much quicker if the Biostrip 42 Rust Remover is warm. Once the rust has gone parts need to be rinsed with water, dried and are then ready for re-paint. Alternatively Biostrip 20 Rust Remover can be used. This is a thick gel which is painted over the rusted areas and left until completely dry. This converts the rust to iron phosphate as before which can then be painted over.

• If using acidic material is a problem, then rusty areas can be coated with Biostrip Rust Remover Gel. This is a safe neutral (non acidic) rust removal compound which is very effective at removing the rust, especially in those cases where the surrounding paint is not removed, as the product will not affect any surrounding paint finish. This is useful where e.g. rust spots are to be removed froma car body without having to strip off all the car body paint first!

• Chemically convert the rust to an iron complex using Biostrip 20 Rust Converter. This is a thick gel that is painted over the rusted areas and left to dry. The advantage of Biostrip 20 Rust converter is that as it dries it converts the rust into an iron complex which resists further rusting, and also covers the complex formed with a layer of transparent polymer, which goes a long way towards protecting the metal surface from further rusting. Thus the resistance to further rusting of surfaces treated with Biostrip 20 Rust Converter is much greater than for surfaces treated with Biostrip 42 or Biostrip 20 rust remover.

Hopefully this covers most of the "how to" remove paint and rust from metal bits.
Summary of Biostrip Products used in this post:

• Biostrip 20 - thick paint stripping gel
• Biostrip 42 - thin immersion paint stripper
• Biostrip 42 Acidic Rust Remover - immersion phosphoric acid based rust remover
• Biostrip 20 Acidic Rust Remover - acidic paint on rust remover gel
• Biostrip 20 Rust Remover Gel - non acidic, neutral paint on rust remover gel
• Biostrip 20 Rust Converter - paint on rust converter/primer