hls30.com Posted February 13, 2006 Share #1 Posted February 13, 2006 Here is something I found interesting, and thought you guys might too. It is a "shade tree test" of whether to use filler over or under epoxy primer. The one real reason I held for using filler over epoxy primer was to seal out any water that might get into the filler from wet sanding. If the filler is not on the bare steel, the water it could hold would not be either...I realy had not considered adhesion as a consergn. Anyway...I am anticipating some good replies... here is the direct link, http://www.autobodystore.com/filler_&_epoxy.htm Here is first part of the blurb-if it all makes it-the second part will immediatly follow in the next post. <TABLE style="BORDER-COLLAPSE: collapse" borderColor=#800000 cellSpacing=0 cellPadding=8 width="96%" bgColor=#c0c0c0 border=3><TBODY><TR><TD width="100%" colSpan=2> The Saga Many discussions arise around whether or not to put filler over epoxy. Some say, epoxy manufacturers actually support this claim, that filler over epoxy is better. The reasoning behind it is that the filler/epoxy/metal bond is stronger and more flexible (epoxy has more flex than filler…) than the filler direct to metal… I wondered in earnest which method was better. Someone on the Autobodystore news group suggested that someone does a controlled test….well I did. </TD></TR><TR><TD width="100%" colSpan=2> The Test I was doing filler work on a truck one night and I decided that while the filler was curing I would go ahead and prepare the test. I cut small coupons of 22 ga, satin steel. The coupons are about 7 by 9 inches…I prepared the coupons as follows: Prepared with a 36 grit Roloc disk (my usual way of prepping filler) Prepared with 80 grit, hand sanded 3M stickit gold. Prepared with 180 grit, hand sanded 3M stickit gold. Prepared with 80 grit + one coat of epoxy. Prepared with 180 grit + one coat of epoxy. I then prepared a single big batch of filler and slapped about 1/16 in on each prepared coupons, pressing the mixture on the coupons well…All of the coupons were ‘filled’ while the filler was still in it’s liquid/plastic state, I was far from the hardening point. The epoxy was cured ½ hour with the help of a medium wave UV lamp. I set it up such that the coupons were warm to the touch, not hot…I wanted to make sure the epoxy was still in its recoat window (7 days at 70 deg f…). Also note that I did not scratch the epoxy prior to putting the filler on it. I followed PPG’s instruction of putting filler on the epoxy within the recoat window. The coupons were left to cure inside my house for a little more than one week with filer and all. </TD></TR><TR><TD width="100%" colSpan=2> The Products Epoxy is PPG’s DP90-LF with DP-402 catalyst. The epoxy was given a 20 minute induction time even though it does not require it. The pot life of this epoxy with the 402 catalyst is 8 hours, so the 20 minute induction time is not a problem… The filler is Rage Gold. The metal is 22 ga, satin steel. </TD></TR><TR><TD width="100%" colSpan=2> The Torture To gauge the adhesion of the filler, I did 3 destructive tests with the coupons. I tried to figure out a way to simulate the worse in car hit for a filler repair. I figured it was an inside out hit. Hitting the metal from the inside forcefully. So I found me a table corner I could spare and gave each coupon a good whack from the back…the results were interesting… Then I proceeded to bend them. In this bend test, I supplemented it by trying to remove filler pieces with my fingers and removed anything that peeled easily… Then I finished with a screwdriver. A flat screwdriver scratching the filler and trying to pry the filler off the metal…. </TD></TR><TR><TD width="100%" background=be.gif colSpan=2> The Results The whack test… I tried to give each coupon a good but equal whack on the table corner… The metal was not kind to my work bench corner…but it was all for the advance of science…So here is how the test went, blow by blow… </TD></TR><TR><TD width="100%" background=be.gif colSpan=2> 80 grit + epoxy Whowa! The first whack a piece flew off…So much for the extraordinary bond of epoxy…Notice that the bond failed in the epoxy, not in the filler. The filler piece still has the epoxy bonded to it (black backing of the removed piece). </TD></TR><TR><TD width="100%" background=be.gif colSpan=2> 180 grit + epoxy Again a piece flew off...Actually 2 pieces…was predictable in view of the 80 grit example…note that the epoxy is still attached to the filler… </TD></TR><TR><TD width="100%" background=be.gif colSpan=2> 36 grit direct to metal Decidedly, whacking it from the back was quite a torture. The 36 grit prepared coupon failed but not as badly as the epoxies. It is not obvious from the picture but the break is not a clean one. There is still some filler stuck to the metal in the area where the piece is missing. The filler failed here, not the bond to the metal…Interesting… </TD></TR><TR><TD width="100%" background=be.gif colSpan=2> 80 grit direct to metal Decidedly, I am lucky I did not bet the farm on the result of this test! The 80 grit did better than the 36 grit. Nothing flew off… </TD></TR><TR><TD width="100%" background=be.gif colSpan=2> 180 grit direct to metal This is the surprise of the bunch…it’s the one that did the best on the whack test! Go figure…I would have never predicted that…The funny thing is that the results are gradually better, 80 did better than 36 and 180 did better than 80… </TD></TR><TR><TD width="100%" background=be.gif colSpan=2>If I can add my general observations on this test is that the direct to metal did much better. The bond with the epoxy looked like the epoxy broke in two. There was still epoxy in the scratches on the metal and epoxy on the pieces that delaminated. This confirms my intuition that the epoxy was the more flexible of the two solids but also the weakest. The filler that failed direct to metal failed in the filler itself. It did not delaminate cleanly like the epoxy did. It left a layer of filler stuck to the scratches in the metal… My conclusion is that the filler is much harder than the epoxy which was the weak point in this test… </TD></TR></TBODY></TABLE> Link to comment Share on other sites More sharing options...
hls30.com Posted February 13, 2006 Author Share #2 Posted February 13, 2006 <TABLE style="BORDER-COLLAPSE: collapse" borderColor=#800000 cellSpacing=0 cellPadding=8 width="96%" bgColor=#c0c0c0 border=3><TBODY><TR><TD width="100%" colSpan=2>The bend test… The bend test is pretty easy. I just bent them in two to about the same angle… </TD></TR><TR><TD width="50%"> </TD><TD width="50%"> 80 grit + epoxy Well, the epoxy did not do better here than with the whack test. This large piece of filler I delaminated with my fingers…Once the bond was compromised lifting with my finger would take large filler pieces off…. </TD></TR><TR><TD width="50%"> </TD><TD width="50%"> 180 grit + epoxy This one, predictably, did worse than the 80 grit epoxied coupon…Again, note that the epoxy to filler bond is very good… </TD></TR><TR><TD width="50%"> </TD><TD width="50%"> 36 grit direct to metal Well, this one did very good. I just could not snap any pieces off with my fingers. I had to take a screw driver to pry some pieces off…The bond was very good… </TD></TR><TR><TD width="50%"> </TD><TD width="50%"> 80 grit direct to metal Again, the bond was much better than with the epoxy but not quite as good as with the 36 grit…but still very strong. Notice that even when some pieces delaminate, they leave some filler behind in the metal scratches…. </TD></TR><TR><TD width="50%"></TD><TD width="50%"> 180 grit direct to metal This is a surprise. It is surprisingly strong. I would even go as far as saying “STRONG ENOUGH!†Gee, this filler does not need much to hang on… </TD></TR><TR><TD width="100%" colSpan=2>The bend test simply confirmed what the whack test revealed. The epoxy was surprisingly easy to delaminate. One thing this test confirmed in my book is the ability of filler to chemically bond to DP epoxy. All of the pieces of fillers still were attached to the epoxy (they had the black epoxy backing). In this test, I did not scratch the epoxy at all, the filler was applied directly to the epoxy within the recoat window as per the PPG P sheet…</TD></TR><TR><TD width="100%" background=be.gif colSpan=2>The screw driver scratches… The screw driver scratch test simply confirmed what the other tests had revealed. The funniest thing is that I could remove most of the filler over the epoxy…not difficult to do with a screw driver… That’s pretty bad… In contrast, the 36 grit roloc, direct to metal prepared coupon was tough as nails… </TD></TR><TR><TD width="100%" colSpan=2> In conclusion… Well, that settles it for me…I will continue to put my fillers direct to metal. The bond was much tougher, even with the 180 grit scratch which was surprisingly tough. Actually, I would say that the 180 grit scratch was tough enough! I don’t believe that it would ever delaminate in a normal auto body situation. It did very good (the best actually) in the whack test and was very difficult to delaminate with the screw driver… I believe that the weakness of the epoxy would have been multiplied with two coats. The thicker epoxy would have been just a thicker layer of weak material. It was interesting to note that in both cases, it was not the bond that failed but the material itself. Both the epoxy and filler were properly adhered to the metal. When the epoxy delaminated, it left quite a piece of itself in the metal scratches. It did not come off clean. This tells me that the epoxy layer was ripped apart. The same was evident with the direct to metal, but interestingly, the layer of material that was left stuck to the metal was much thicker with the direct to metal filler… To me, proof is in the pudding…no wonder I never get a filler failure when applied direct to metal…that stuff sticks! </TD></TR><TR><TD width="100%" bgColor=#ffffff colSpan=2>Serge is a regular contributor to the Autobodystore.com BBS and has had extensive experience with both body repairs and painting. His results should be recognized as a reflection of this experience and his ability to evaluate the materials and process he used. Different materials and processes could result in different or similar results.</TD></TR></TBODY></TABLE> Link to comment Share on other sites More sharing options...
26th-Z Posted February 13, 2006 Share #3 Posted February 13, 2006 Thanks Will. Good article. Link to comment Share on other sites More sharing options...
ncz Posted February 13, 2006 Share #4 Posted February 13, 2006 Will, good find!!! Now can we practice on your car!! Link to comment Share on other sites More sharing options...
hls30.com Posted February 13, 2006 Author Share #5 Posted February 13, 2006 That is pretty much the plan!Will Link to comment Share on other sites More sharing options...
EScanlon Posted February 13, 2006 Share #6 Posted February 13, 2006 Well that confirms something I thought I "knew" and had become ambivalent on. Excellent piece Will.E Link to comment Share on other sites More sharing options...
260DET Posted February 13, 2006 Share #7 Posted February 13, 2006 What is the 'filler' he was talking about? Polyester? Another relevant consideration is that epoxy does a lot better than polyester in sealing against moisture. Consider also whether the preparation of the metal was suitable for epoxy. Points for discussion, body work is not my field of expertise Link to comment Share on other sites More sharing options...
LanceM Posted February 14, 2006 Share #8 Posted February 14, 2006 It was decided on hotrodders.com that the test is flawed. I will see if I can find the original thread there and link it. Link to comment Share on other sites More sharing options...
LanceM Posted February 14, 2006 Share #9 Posted February 14, 2006 Here's the linkhttp://www.hotrodders.com/forum/filler-over-epoxy-epoxy-over-filler-here-some-good-info-74668.html?highlight=epoxy+primer Link to comment Share on other sites More sharing options...
hls30.com Posted February 14, 2006 Author Share #10 Posted February 14, 2006 Thanks Lance! Link to comment Share on other sites More sharing options...
shamus11 Posted May 7, 2006 Share #11 Posted May 7, 2006 Don't you hate it when searching for answers one something you were 99% sure of leaves you more unsure!I've sprayed my stripped back panels with an etch primer. I'm going to do the whole car like this first before I bring out the filler. I'll probably heavilly scuff the panel with 80 grit before applying the filler but won't take it right back to bare steel again.I like the security of a rust preventative coating directly on steel. I've been told 2-pac primers are also porus. Link to comment Share on other sites More sharing options...
madkaw Posted May 7, 2006 Share #12 Posted May 7, 2006 I thought it was interesting that you bondo(their bondo) right over the por-15 rust treatment before it even cures. If you haven't been on their site it is worth a look. Link to comment Share on other sites More sharing options...
Jeff Berk
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