skin seam sealant

[tgmillso]

Hi All,

My question is around seam sealing after priming. It is common after priming of a car to add seam sealant (like polyurethane Sikaflex 227) along edges to bridge the gap for top coating and prevent moisture egress. These can be used over any primer, and can be topcoated with PU. I fay sealed/wet riveted the lower skins of my fuselage, however not my wings or top skins of the fuselage. I am thinking that I would run a small seam along the edge of an overlapping skin, or where there is a butt joint, such as along the main spar where corrosion is common on certain types of certified aircraft. I have noticed on certified aircraft that there can be a significant gap between the overlapping skins (especially where 1/8" rivets are used), leaving the joint susceptible moisture entry. Has anyone done this (where they haven't fully fay sealed in a traditional sense), and are there any downsides (apart from time)?
Cheers,
Tom.

[tgmillso]

I used the flap as a test case yesterday. It had been primed with Wattyl Super Etch the day before. I called Sikaflex tech support and they confirmed that 227 should be fine directly over the etch primer, and in turn the PU can be applied directly over the Sikaflex. I filled some MK319-BS rivets with it, then ran a very fine bead along the bottom skin seam. Because the skin is so thin, this seam is extremely fine and I suspect that the topcoat will bridge this quite adequately without the seam sealer, and that the seam sealer will be more appropriate along the top of the main spar and some of the thicker fuselage skins but I wanted to test it on this smaller surface before I went full speed on the fuselage. I have left things to cure for a full day before top coating, as the full cure speed is approximately 2mm per day at current temperatures, although in the spec sheet it says that top coating can be performed as soon as skinning has occurred.
Tom.

[rv6builder]

The mechanics at the float operation I used to fly for would mask off a thin strip along each skin seam, then apply a thin bead of proseal along it to seal the edge. Not the most streamlined, but I plan on doing that to my fuselage as my RV will probably be staying outside for the foreseeable future.

Alex

[longline]

lap joints is a good thing. I have been thinking about the structures that have previously been built and think I have a decent suggestion.

There are some products that are very low viscosity that can be applied to the lap joint that will wick into that joint interface. They are pretty good at sealing things up, as they use this in the tank areas on helicopters.

I will try to get the information on this product, as I cannot dredge up the name of the stuff right now.