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wrote:
My friends 43'aluminum houseboat has developed some severe pitting on
the bottom. Since he bought it, it has been in a lift and will continue
to be - the pitting existed before he bought it. Some of the hull is
like new but bad pitting where there has been barnacles.

We are having it hauled and sandblasted Mon. The yard also has a
treatment which they guarentee will remove any trace of salt in the
aluminum pores. I expect we will find too much pitted area to
economically weld.

Question - how well will expoxy impregnated fiberglass mat adhere to a
well prepaired rough surface ?? Patches would be 1 to 2 ft. sq. Boat
does 8 mph max and we would be happy with a 10 year life to the
repairs. The longest the hull will be in (salt) water will be 1 week
and then back in the lift. BTW we will be painting with 3 coats of
epoxy paint after repairs.

Glassing over a freshly blasted aluminium surface is a common treatment
for badly corroded aluminium boats here down under.

Welding on tingles is not usually a success, if the hull is corroded
that badly it will have recurrences regardless, so you need to either
replate the complete bottom (sometimes viable) or cover it with glass as
already suggested.

Below is a paste of one of my old posts on the subject.

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Robin,

You don't mention how old the hull is so I'm obviously guessing,
but these "long cracks" usually start appearing as the hull bottom in
general starts to lose it's integrity from corrosion. Small cracks can
usually be welded, but long runs such as you describe are caused by too
much flexing so the crack is only the symptom of a wider problem.

Obviously welding in new bottom plates, despite the extreme expense
related to an old hull can help, but doing only one side can sometimes just
aggravate the rest of the corroded bottom, so the whole thing will need
re-plating anyway.

This might not suit your situation, but even so it might still be
of general interest to resuscitate old aluminium hulls. It may (probably
will) be criticised but is regularly done here on commercially used salt
water tinnies, which can have their service life (at least until a complete
bottom re-plate) significantly extended.

(i) Have the bottom grit blasted to provide a good "keying" surface
(you'll be amased at the corrosion that was not previously apparent)

(ii) Without delay, have any sharp corners (around keel, spray guards
etc.) filled & smoothed with fibreglass filler (Q cells in polyester resin
are fine)

(iii) Then apply two layers of chopped strand mat over the entire bottom.
(again; cheap easy polyester resin works fine but if your supplier can
supply one of the slightly more "flexible" polyester resins all the better
but not absolutely necessary)

(iv) The glass covering usually finishes at the chine & if done
immediately after the grit blast & the al./glass junction is finished over
with filler it will have a good (no delamination) service life.

(v) The bottom can then be surfaced finished to the desired level,
(although in hard commercial use not too much) anti fouling, whatever..

(vi) Yes the boat is a bit heavier (but don't forget fibreglass below
the waterline, displaces half its weight) & it's not as abrasion resistant
below the water line, but it's cheap, the boat's strength is back & no
leaks.

(vii) The boat can always be re-plated further down the line, or the
glass if damaged can be repaired in the normal manner.

Karen Smith.