Clarify "mating surfaces" please

OK - I am about to start an RV-12 (kit is on order). I've read many of the priming posts. Priming "mating surfaces only" seems prudent, using simple rattle-can self-etching primer as an effort-vs-results tradeoff. BUT - please clarify exactly what is meant by "mating surfaces" in these circumstances and the additional questions below!

True, or false, or explain further please!!

1. Any non-alclad part gets primer. This is accomplished by cleaning with acetone, then spraying with self-etching primer. No scotchbriting of the surface is needed, because the primer is self etching.

2. When a primed non-alclad part is riveted to an alclad part, no priming of the alclad part is needed. Correct?

3. If you use the scotchbright wheel technique to debur a row of holes on an alclad part (on both sides), does this remove the alclad so that the surface needs priming before riveting? (Obviously it will get primed before painting the topside) I ask because I don't seem to see that practice on most builder websites (the "panels" of blue plastic left in between the rows of holes don't have any spray on them, as such priming would produce.)

4. Ribs are alclad. Does the process of fluting the flanges and otherwise adjusting and deburring them remove the alclad? Assuming you are priming "mating surfaces only" should rib flanges be scotchbrighted and primed? (Or omit the scotchbriting if using self etching primer?) SHould the underside of the alclad wing skin similarly be scotchbrighted and primed (just along the holes) before riveting to the rib flange?

5. For alclad material in general, how does deburr of holes and edges, as well as edge breaking, affect the alclad?

6. Are their nuances I have missed? Do I have some basic misunderstandings?

Thanks! Bill

Welcome to VAF!!!!

Bill, welcome to VAF

Sorry, cant help with your question, but I am pretty sure you will be getting an answer or three soon.

Good luck with the project, enjoy.

Welcome Bill...

Mating surfaces- where any two pieces of aircraft structure make contact with each other.

1. Always roughen any surface(red scotchbrite pad or 320 grit aluminum oxide sandpaper not to be confused with scotchbrite wheel) before applying self etching primer so that it has something to grab a hold of. Then clean before applying primer.

2. Not unless you want to or if it is going to be in a location where water/debris may collect and cause corrosion.

3. I would not recommend using scotchbrite wheel for deburring...you will most likeley go through the alclad coating(2.5-5.0% on each side) On thin skins you can put a leather glove on, pass it over both sides and your done. If you use the deburring tool you will enlarge holes in thins skins(.025" and less).

4. Fluting/straightening process does not remove enough of the alclad to worry about. Prime mating surfaces if you wish.

5. Same as 4.

Keep searching/asking questions. If your plane is going to be hangared and away from salty ocean air then you may choose not to prime many of the alclad surfaces. Saves your health, money, time and weight. I have used about 5 cans of SEM self etching primer to date. Have fun.