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.
