Silicone sealants and rust

[Mforcer]

It has been mentioned previously that the use of silicone sealants on painted metal can cause rust and the recent discovery of a silicone like sealant on my car has concerned me. After some reading I have found that there are two types of silicone sealants; neutral (non-corrosive) cure silicone and acid curing silicone.

The acid curing silicone is corrosive and would etch into the paint and ultimately cause rust. This is usually designed for use on glass.

The neutral cure silicone is non-corrosive and I assume will not etch into the paint surface and should not cause rust. This is the type of silicone used for metal roofing etc. and one manufacturers website lists the uses including "Weatherproofing caravans and trucks".

So, is the use of a neutral (non-corrosive) cure silicone sealant bad for cars?

[charlottesbutler]

I have had similar concerns after reading comments elsewhere about corrosion. Cheap handyman sealants with acid cure do seem like a bad idea. However, I have used Sikaflex for more than 20 years on houses, cars and the most corrosive environment of all - saltwater yachts - with no problems whatsoever, except that it is really tough to remove once fully cured. Some degreaser and polish stripping agents may not be good for paint, nor react well with the sealant. Sika recommend isopropyl alcohol, which is not widely sold. Bunnings or a boat chandler may help. Note that several different acid free sealants are offered, for water tanks, grey water, glazing etc.

Why not use the free call number to get advice from Sika direct. I am sure they will not mislead you. Tel: 1300 22 33 48
You could also ask an older but uptodate panel beater, if you can find one. I hope you will let everyone know here, if you get an answer that convinces you.

Two links which you may already have seen:
http://www.sika.com.au/cmc/Datasheets/tds/Sikaflex11FC_tds.pdf
http://www.ehow.com/way_5661265_tips-working-sikaflex.html

Rod
(76 450se, dull red!)

[oscar]

+1 for sikaflex. The stuff I've used, automotive one for the "leaking cabin" fix, was polyurethane based. The fix was via wire brush, sanded, etch primed then the polyurethane squeezed from a tube and applied with a gloved hand and it covered primed and non primed areas.  That was in 2005 and considering my 350 sits out in the rain a lot, plus used to get washed a lot, problems would have arisen again by now if the sikaflex was corrosive I reckon.  Not sure which automotive sikaflex it was though.

By recommendation, instead of using mastik for windscreen sealant I bought a sikaflex product, again not sure which product, but polyurethane based.  My son and I are building canoes at the moment and using sikaflex 291 as a sealant which is suitable for marine apps.  Though I wouldn't expect a plywood canoe to rust, the sikaflex is again polyurethane.  All their product packages are fairly well descriptive in what the product can and can't do.  I bought the stuff from bunnings but SuperCheap has some marine plus a few auto Sikaflex products.

I'll admit I have used your average type handyman silicone about 5 yrs ago for tailight seal, squeezing it in above the lense and anodised chrome piece.  Just checking now and there appears to be no damage.  Just the clear silicone becoming more opaque but not touching any paint either and I've no idea if it was acid cure or not.

To summarise, just use a sikaflex product 

[TJ 450]

Yes, it is the acetic cure products that cause rust.

There shouldn't be any problem with using polyurethane products, and indeed Sikaflex is a reputable product line.

Tim