Re: Please help, 1st time painter

Posted by Rod (NH) on August 27, 2005 at 17:44:35 from (64.140.200.138):

In Reply to: Re: Please help, 1st time painter posted by CNKS on August 26, 2005 at 20:16:53:

The 180 or coarser (120-180) is definitely on the tech sheets for both the 170 in the OMNI line and the DPLF (80-180) in the Deltron line when either is applied to bare sheet steel. That essentially means that a surfacer would need to be used all the time, even on brand new sheet steel panels that do not require any additional bodywork. Maybe it's just me, but I dislike the thought of taking 180 to nice smooth steel just to make the paint stick, especially with a product (epoxy) that is known for great adhesion in the first place. I have no doubt that if anyone did comparative physical adhesion tests with 170 on steel prepped with both 180 and 320 grits, no other treatment, that the 180 would win the contest for better adhesion of the two. That doesn't mean that the 320 will not work, only that the 180 would be better. As I see it, this boils down to "how good does it have to be to be considered good enough?". The safest action is the 180, but I don't think it is always necessary.

On new (or excellent condition) steel with no rust, after degreasing with DX330, I've just scuffed with a maroon (general purpose) Scotch-Brite pad, applied two coats of 170 and topcoated directly with MTK. No hint of any "sandscratches" and no later problem with adhesion, at least after three years. The maroon Scotch-Brite grit equivalent is 360. I suppose if I knew that a particular part was going to be subject to particularly rough treatment, like abnormal flexing or something, I would probably choose to go with the 120-180 + surfacer per the tech sheet. Mostly though I would not, unless I knew beforehand that surfacer was going to be needed anyway because of dent or pit repair or something else requiring grits that coarse.

Some time back, on Autobodystore.com, I recall seeing the results of adhesion tests for application of polyester filler (Bondo). The issue was whether it was "better" to apply the filler directly to bare metal or apply epoxy first and then the filler. Opinions differ sharply on this, just like they do on epoxy vs etch primer. The conclusion was the application of the filler to bare metal was better than to epoxy, all other things being the same. That is, of course, something that I rarely ever do. For unrelated reasons I much prefer applying the epoxy first and then the filler. I have found that method to be perfectly satisfactory, even though it may not be "as good". In other words, I accept the fact that it's "best" to apply filler directly to metal from a strict adhesion standpoint but I remain unconvinced that the difference is serious enough for me to change from my preferred method. I think a similar reasoning can apply to the rather heavy sanding of otherwise smooth and clean sheet steel, especially when an epoxy is going to be used.

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