A "toothy" dillemma

I am preparing to barrier coat my hull with epoxy. I have glassed it with epoxy and faired with a DA sander using 40 grit discs. Unfortunately, this has left tiny ridges of "fur" where the stitching in the biax has been sanded. I can get rid of them by sanding with a finer grit sandpaper, but I wonder if this will leave enough tooth for the epoxy barrier coat. All the references I have ever seen recommend 80-100 grit for tooth, yet this still leaves ridges at the stitch lines which will bugger my barrier coat.
I am currently running a little experiment in which I sanded portions of the hull with 80, 120 and 240 grit, then applied a small strip of glass to each with epoxy. I will then see if there is a noticeable difference in adhesion between the different grits, but I thought I would put this question to the group as well.

Mike

 

You haven't enough epoxy over your 'glassed hull. If this is biax or triax, then it'll print through like a mother . . . without a substantial layer of more goo. Naturally, not sanding through this goo, before you barrier coat is key. You don't need a 40 grit tooth, 100 is fine. Much more the 150 and you're getting too smooth.

You need at least 10 mils of epoxy (minimum) over your 'glass work or it'll print through and you run the risk of dinging the knitted fabric stitching, as you sand. I usually grind off the stitching before over coating, to make it easy to fill the weave and build film thickness. The stitching is just cheap cotton string and imparts no strength to the laminate.

 

Most stitching is polyester yarn, but for the rest I agree.

 

The results of my experiment were interesting, but not surprising. All of the strips adhered very well. I could not pull any of them off. Which begs the question, is the 80-100 grit recommendation merely dogma? Are finer grits just as likely to create a strong mechanical bond? I have never seen any hard evidence other than my very limited and flawed experiment but the results would seem to indicate that grit size is not so relevant. This makes sense in that a mechanical bond occurs at the molecular level and would be more influenced by total surface area than the depth of a groove. I don't suppose the epoxy sees much difference between a few deep grooves or many shallow grooves.

It would be interesting to run a better experiment using a larger number of substrates and glues and using a tension gauge to measure peel strength.

Mike

 

Actually, tests have proven that the depth of the tooth grooves does make a difference, as it's this extra surface area that gets the job done, not to mention the mechanical "keying" of the rather large epoxy molecules. I'm not sure of your testing procedures, but most back yard testing is quite subjective and typically without even a base line. For example, how did you measure the peel strength of the various areas? I'll assume you didn't have a load cell laying around looking for something to do, but maybe a spring style fish scale could relieve some comparable information.

I wish you were right and I could just buy one grade of paper and be done with it.

 

Par,

I would love to see the results of those tests, do you recall where you saw them? You are right about my test of course, it was very crude. I was unable to pull any of the test strips off so was unable to establish that there is any difference in adhesion between grits.

Mike

 

Go over to Westsystem.com and have a look at their data.