|THE MINI MAG. Volume 2 No.8|
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|There are several reasons to strip the paint (and any rust) from your body shell: rust repair, scratch/paint damage repair, or for re-spraying as part of a restoration. There are also many techniques, each of which has advantages and disadvantages. Choosing the wrong stripping method can be a mistake. For example, if you get your car bead blasted and are not ready to paint, you will find yourself with a car-shaped piece of rust in a very short time. The summary below shows the major paint and rust stripping methods, with their suitable applications, advantages, and disadvantages.|
Method: Acid/alkaline dipping
Entire shell is dipped in acid or similar chemical, and all rust and paint is removed by chemical reaction. The body is then ready for panel beating or painting.
The most expensive car stripping method, but also the most effective in removing absolutely all rust and paint.
The newly exposed bare metal will start to rust within a very short time unless it is primed. The company will usually prime or phosphate the shell to prevent immediate surface rusting. They may oil the shell or provide some other temporary coating that must be removed before painting.
Box sections and seams can trap acid/stripping fluid which may seep out over a period of months or years, ruining new paint work, though I have never seen reports of this actually happening. The company should water blast or chemically clean the shell to prevent this. It is wise to open up box sections to allow them to be cleaned and rust proofed. I have personally seen rusty box sections discovered on a recently dip-stripped car - it might pay to use a borescope to inspect all crevices and box sections to ensure they have been adequately stripped and painted/protected.
Method: Chemical stripping
Paint stripping solution is applied to the paint and left to stand - paint is then scraped off.
Leaves bare metal, but dissolves body filler.
Time consuming and unpleasant but highly effective.
Difficult to clean out nooks and crannies that become filled with stripper and difficult to remove dissolved paint from complex surfaces such as castings and Mini roof gutters. This leads to the possibility of stripping chemicals becoming trapped in seams.
Good for large, flat areas, e.g. body sides and roofs.
Method: Machine sanding
Paint is sanded away using a grinding disc.
Slow but effective.
Risk of warping panels with heat if the grinder is held in one place for too long.
Risk of leaving metal scratches that will show through paint if the surface is not adequately sanded using a finer-grid disc of at least 80-grit after initial sanding.
Impossible to strip areas that are not almost flat.
Sanding dust is poisonous and gets everywhere.
The method of choice for sanding out scratches/surface rust for spot repair.
Method: Sand/media blasting
Sand, or other media (crushed walnut shells, baking soda, glass beads, etc) is sprayed on the panel to remove paint, rust, and sometimes filler.
Highly effective and fast, removes rust.
Sandblasting is not recommended. It work-hardens the panels making them hard to panel beat. Body panels will warp if both sides are sandblasted, or if the operator is untrained (many sandblasters are used to blasting industrial equipment and bridges, not 1mm steel) and spends too long in one area.
Other media apart from sand are gentler to panels.
Rust particles mixed with paint flakes and blasting media get in every nook and cranny, the car must be very thoroughly washed and dried a few times to remove it, or the rust will be trapped under the new paint and start again.
Seams should be avoided if possible as rust and blasting media will become lodged in them. However, there are few methods as effective for getting rust out of Mini roof gutters.
The car must be painted immediately after stripping to prevent rust. The shell must usually be transported to the blasting premises. These two factors can prove to be very inconvenient.
A popular method for stripping an entire car.
Does not remove underseal.
3M Clean-N-Strip discs used in an electric drill to strip paint and rust without removing any panel metal. Wire wheels can be used to get into areas too small for the Clean-N-Strip disc. Comments
Gentlest way of removing rust and paint, but better suited to small areas.
Difficult to get the disc into confined spaces or near flanges.
Recommended for stripping surface rust from floor pans or similar mostly-flat areas that are not too large.
Scuffs the surface of the metal to provide good paint adhesion. Wire wheels tend to polish the metal surface, and therefore may not remove as much rust.
Choosing a Paint Stripping Method:
To strip the paint on a Mini, I recommend chemical stripping for the home restorer, with disc sanding or hand sanding for any tricky areas - be careful to thoroughly remove all stripping compound from areas such as the roof gutters.
If the budget will allow professional paint stripping, media blasting or acid dipping are excellent; acid dipping has advantages but it usually more expensive. Ensure that the car will have a coating applied to it that will prevent rust until such time as you are ready to paint it. Typically, the car will be phosphated (metal conditioned) and will need to be primed within a few days.
Do not attempt to strip the entire car at once unless it is absolutely necessary and you are working fast. Instead, strip the roof and phosphate/prime it, then do the same for body sides, front/rear and the interior, if you are stripping it.
If you have access to a competent media blaster, mask the inside of the car and have them strip out to within a few inches of the seams, then finish the intricate areas yourself. Get metal conditioner, etch primer and primer onto exposed areas as quickly as possible, to prevent rust.
If the paint on your Mini is in good condition, seriously consider not removing it all, especially in the interior which rarely receives new paint and hence should not be too thickly covered. Instead, clean up any areas of rust, scratches and other damage to the paint and prime those areas.
When to Strip Paint:
The original paint film must be assessed to determine whether it is safe to paint over. It should be removed if:
It has crazed or oxidised (become chalky) or has water/air blisters.
It reacts with the paint to be applied.
The paint is too thick (the car has been repainted too many times already).
The owner desires a "bare metal respray".
The car is going to be painted a different colour. This is not strictly necessary but prevents the base colour from showing through in worn/chipped areas.
Often, if the paint is sound, there is no reason not to paint over it. Any scratches or patches of surface rust can be primed over and the entire surface flatted back before the car is resprayed. This avoids the hassles of stripping and the risks of surface rusting while the metal is bare.
On the other hand, if you suspect filler or bad repairs, removing the paint is the only way to find out accurately the condition of the shell. Also, if a complete respray of the shell is undertaken, every little nook and cranny must be flatted back so the new paint will adhere to it.
Getting to Bare Metal - What to Do.
Before you start stripping your car by your chosen method, clean it thoroughly and then clean it with Prepsol, a product that removes wax, grease and silicone - three things that do their best to stop new paint sticking to metal - from your paint. Strip the car and apply Deoxidine to bare metal. This metal conditioner reacts with the bare surface to give an iron phosphate coating that is highly rust resistant and provides an excellent surface for primer. Spray with the etch primer and primer of your chosen paint system as soon as possible as metal conditioner does not protect metal against rust very well by itself.
You are now ready to do any necessary body work. Depending on environmental conditions the primer may hold out against rust for a period of days to a year or more. The former estimate applies if you live by the sea and store the car outside, the latter if the car is stored indoors in a desert, in a Carcoon-type cover, or in a sealed garage fitted with a dehumidifer, and good quality primer is used.