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Danger to underlayment using light torch on flashing?

Posted by fixizin (My Page) on
Thu, Jan 28, 10 at 16:55

This is a So-Fla HVHZ concrete tile roof, but I'm sure the principles apply elsewhere. The galvanized edge flashing was too new, and not properly etched, before an expensive primer (designed/labeled for just this application) was applied.

All LOOKED well for 21 mos. until Hurricane Wilma came through, and gave it a good sandblasting. It's looked very "poxy" ever since. FINALLY getting around to addressing this... (hey, gimme a break, it's cosmetic, and it's not visible from the street. ;')

I supervised this roofing job back in 2003, so the details have faded a bit. IIRC, the L-section flashing was nailed down after the felt, but before the hot-mop asphalt layer. Per newer codes, there's a 1x2 strip that spaces it out from the rafter ends/sheathing edge.

Anyway, I want to strip the old primer, rub the bare flashing with white vinegar to micro-etch the surface, and have another go at the primer... maybe even follow up with paint this time, LOL.

Was contemplating messy chemical stripper, when someone suggested ye olde propane torch. Sure enough, a few seconds of even the cooler tip of the flame, and the primer scraps off "like buttah". Did a tiny test section, didn't smell anything like burning tar... so, is it a roof-safe method?

Also, I remember that roofers use propane to melt the hot-mop tar (570degF, IIRC); add to that the not-great heat transfer of thin sheet metal, and the dissipation offered by the adjacent cool sections, and I think I've got "margin" to play with.

BUT... wanted to bounce it off the RKIs/experienced DIYers here to see if any yellow/red flags went up!

Thanks in advance.


Follow-Up Postings:

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RE: Danger to underlayment using light torch on flashing?

Use a wire wheel in a drill.


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RE: Danger to underlayment using light torch on flashing?

Galvanized metal must be specially treated and painted in the factory immediately after application before it can oxidize. The longer you wait to paint it, the less likely the paint will adhere contrary to popular lore. Removing the oxidation later is difficult and rarely worth it but not impossible. Google it to find some procedures.

Removal of paint can be done with a hot air device made for that purpose. An open flame in the hands of an amateur is never a good idea.


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RE: Danger to underlayment using light torch on flashing?

"Removal of paint can be done with a hot air device made for that purpose."

Hot air guns are powerful enough to melt solder. Around 700 F.

There use on metal that may be against wood is not a great idea.

Paper and cellulose products begin to char as low as 451 F.

It is not that hard to etch galvanized in situ for painting.


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RE: Danger to underlayment using light torch on flashing?

Didn't mention it in my OP, but I already tried Ye Olde Wire Wheel--it just stains the white primer--the stains being the exact color of the wire wheel, lol. That's some serious primer!

But I agree with everything Brick said in his 2nd post.
So will vinegar (5% acidity) do the etching job, or is there something better? Muriatic acid just seems WAY too strong.

The part of the flame I'm using will not even melt lead-free solder with a melting temp. of 425degF, let alone the roofing tar at 570F... I'm gonna go for it... carefully.

PS: I'm no rank amateur with ye olde open flame... I'm an ADVANCED amateur, lol... let you know how it goes. ;')


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RE: Danger to underlayment using light torch on flashing?

"Didn't mention it in my OP, but I already tried Ye Olde Wire Wheel--it just stains the white primer--the stains being the exact color of the wire wheel, lol. That's some serious primer!"

Any primer that tight is fine as it is.


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'Ancient' edged 'weapons' to the rescue, lol...

Well a light and judicious application of the torch did work, but it was causing some warpage of the flashing, which mostly "relaxed" when it cooled... but not entirely. Was especially noticeable at the overlaps.

And of course the torch just loosens the primer, does not burn it off--still gotta scrape, albeit lightly. That's when it hit me, that my Red Devil scraper edges had become almost uselessly DULL over the eons. Took said scraper to my local True Value guru (age 74 going on 61), who restored some wickedly square-sharp edges to that puppy.

No torch req'd--that Red Devil now scrapes on both the push stroke AND the pull, and that old primer is tough, but brittle--catch one edge and it breaks up like an ice sheet.

Now I'll let the bare flashing "weather" for a few weeks, then hit it w/ vinegar, followed by quality primer and paint from Acrylux... worry about it again in 2021.

Thanks for all the inputs. There's just no textbooks for some stuff. ;')


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