I debated this for some time, searched VAF, asked every builder in my chapter and examined many finished aircraft. In the end I decided there was a greater chance of getting a gap in the ProSeal if I dimpled, so I decided to countersink the holes. To prevent the baffle from walking during riveting, I deferred countersinking every third hole until after assembly.
I loaded ProSeal into a Ziploc bag, snipped off the end and piped a bead of ProSeal around the edges like a cake decorator. It took about 100 grams of ProSeal to do the whole baffle. Once I had a nice bead along all the rivet holes, I added some extra sealant around the corners (because this is a common area for leaks) and then dropped the baffle straight down into place. I started clecoing by doing all the ribs first, then I clecoed every third hole, which had not been countersunk. Once all these were in place, I went back and clecoed the holes that were countersunk. I let this set up for a few hours before I began riveting.
I started riveting according to the plans with the top and bottom rib holes, and then riveted the T-712 Z-brackets with pulled rivets. Next I put solid rivets in the sides and then riveted the skin to baffle holes that had been countersunk. Once all the countersunk holes were riveted, I removed the remaining clecoes, countersunk the holes I?d skipped during the skin prep, added a little ProSeal to the holes, and riveted them.
This method kept the parts in alignment well and I didn't have any leaks.