media blasting, dipping, and coating advice

Wes

Well-Known Member
Site Donor $
Messages
1,130
Reaction score
746
Location
Tasmania, Australia
I was lucky, the previous owner bought the last of the original BMW parts, and extra steel, over 16 years ago. It's all sat in a trailer ever since. I did buy some WN inner fender parts, but I'm not building a CSL unicorn :) I'd love to see more pics of your car, beautiful!
More pics on the way soon..
 

eriknetherlands

Well-Known Member
Site Donor
Messages
1,082
Reaction score
511
Location
Netherlands, Eindhoven area
just some considerations

With blasting;
- you do not clean the insides of a seam, nor do you clean the inside of closed, or partially closed volumes.

with e-coating:
- Normal process uses an acid to etch the rust out; this acid creeps into most crevasses by capillary action, but not all.
If the panels are tightly together, then the acid won't enter. Light rust does remain behind in seams, but flaking rust sections do get eaten completely.
The acid used is not always flushed out completely from the seams (in the less professional shops). This acid then leaches out, causing rust issues a few years down the line. It is avoidable; good shops use high pressure washers, and religiously clean every seam from begin to end. There are a few companies that started using something else then acid, which appearently has no risk of damage when leaching out.
- the e-coat is done by electrically charging the body. The bath is then charged oppositely. The current flowing between the two then actively pulls the e-coat onto the steel. However the electric attraction is not evenly spread over the surface of the car. Think about this as the magnetism fields around the earth; at the poles the lines are closer to each other, around the equator they are further spaced apart. In complex 3D structures the same occurs: at certain locations there is a high attraction for the E-coat, but inside boxes the magnetics lines are almost absent; as a result there is hardly any deposition. It is even noticable on inside radiuses. For this reason in automotive manufacturing the E-Coat process is often followed by a dip in a paint bath, covering the surfaces that may not have recieved full e-coat coverage. The company doing the dipping also sometimes has the possibility to stick an electrode in an opening, forcing extra current in specific areas that otherwise would get less or no coverage. This however is not common for classic vehicles as it's too complex for that one e9 that passes through their facility that year. .

note that I'm not in either team "blast", nor in team "dip". I'm in both :)
I drill out the spot welds, pull the panel off, weld the holes shut, blast or sand, then send the seperate panels out for e-coat (zinc plating) and then spot weld everything back together, Best 'as-factory' match as I can showing original spot welds, using maximum rust protection having 2 zinc layers in a seam, but I do loose pencil marks, the overspray pattern that it had from factory and other original marks.
Oh, and it takes bloody long to do all that.

Erik.
 
Last edited:

autokunst

Well-Known Member
Site Donor $$
Messages
2,746
Reaction score
1,657
Location
Milwaukee, WI
Erik,
I am continually amazed and inspired by your work. A true artisan - I hope I can approach the level of your results.

As for original factory pencil marks, these essentially mean nothing to me. Sure, if I had a car that was so solid and original that it still had these marks, I'd probably geek out on it. But I honestly don't think most of these cars are that immune to corrosion. I am finding seams in high up places that even the dipping didn't thoroughly clean. The only way to get to it is to peal it apart, clean it, treat it, and put it back together better than the factory did. In fact, these pencil marks are not the artwork of Wilhelm Hoffmeister, Paul Bracq, or Giovanni Bertone. No, these are assembly marks from Karmann factory workers in Rheine, and maybe some final assemblers in Munich. For me (and my needy chassis) the metal work far outweighs the graphite.
 

tmh

Member
Messages
63
Reaction score
30
Location
Naples, FL
It was my impression with media blasting that the different media used determines how aggressive and abrasive it is. Getting a chassis down to a bare shell to dip is a tough job.
 

eriknetherlands

Well-Known Member
Site Donor
Messages
1,082
Reaction score
511
Location
Netherlands, Eindhoven area
It was my impression with media blasting that the different media used determines how aggressive and abrasive it is. Getting a chassis down to a bare shell to dip is a tough job.
Media does affect surface appearance, but also the heat generated.
Glass leaves a smoother surface compared to korund (alum oxide).

Besides choosing materials (glass, walnuts, plastic, alum oxide) there is also the choice of particle size. Bigger is faster, but gives tougher surface.
 

m5bb

Well-Known Member
Site Donor $
Messages
1,848
Reaction score
607
Location
Peachtree Corners, GA
I'm late in here and probably posted something about this 4 years ago when I started my coupe but will provide a little info.
I'm in Atlanta GA and didn't find a dipping process here at that time.
When I read up on this the "cool" thing to do at the time was Soda Blasting.
I did this and cost about $1200 and no warped panels.
There were a few places they didn't or couldn't get to or inside of.
Body was bare metal though and my painter sprayed it with gray epoxy primer to protect it while the metal repairs were done.
My metal guy did have to remove the epoxy paint in the areas where he had to weld.

Overall it was pretty good. 8 or 9 on a 10 scale.
My interior was coated with a material called Lizard Skin. Actually 2 coatings. One for sound and one for heat. We coated the trunk too. Then painted the dark gray with the body color.
My under body was sprayed with a material like bed liner. You can tint it any color you want. It has a little texture to it.
My car is NOT a restoration but a period correct Hot Rod. So I did what was the best to preserve the car as it gets driven, a lot.
Gary
 

Attachments

Top