Refinishing the seating of a fillet panel with PR1440 being applied over an old (damaged) sealant? 1

[ZeeS]

During the refinishing of exterior of the A/C, after PDM (Major overhaul), damaged sealant on the seating of fillet panels was required to be refinished.… From past experience and as a standard practice, old sealant was required to be completely removed first and then new one was applied, since old and new layers, being long chain polymers, won’t stick……

Recently I heard about another procedure to degrease the old sealant with MEK or PD680, scuff it and apply a fresh coat………..

Did anybody experience this practice in the past? Or knows of any justification? Or any technical answers for this?

thanx

 

[berkshire]

You do not give enough information to answer the question.
If the sealant is a silicone then the answer is no, you cannot do that, you have to take it all off.
If the sealant is a polyurethane then you MIGHT be able to what you are asking.
More details of the type of sealant and/or the aircraft will get you a better answer.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[berkshire]

I am sorry ,I should have said"Existing sealant" Poly sulphides do not stick to silicone.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[ZeeS]

Berkshire!
sorry for not quoting the sealant. Old one is PR1422, where as the new one is PR-1440.

 

[berkshire]

Ok so this is the old PRC loved and hated by mechanics everywhere.
I think you are going to have to get it off, Scrape as much as you can, use an acrylic scraper, then use MEK to clean off the the rest.
Methylene Cloride will also remove it.
The solvents break the polymer chains, making the PRC swell and wrinkle.
I would not put new over old if you have used solvent.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor