First, determine what the "plastic" is !

Chances are, it's fibre-glass - that's a thermoset, and the only way to
repair is to use a fibreglass repair kit - relatively cheap at any motorist
outlet. Do ensure the glass fibre is FULLY covered with the resin, no loose
strands or ends, else you'll get leakage by "wicking" - water seeping out
through the strands of fibreglass.

(You may be able to see whether it's fibreglass - look at the cracks, is
there a white substrate with evidence of "fibre" - that's fibreglass with a
paint job).

Plastic welding will only work on thermoplastics. (difference between
"thermoset" and "thermoplastic" - once set/formed, thermoset cannot be
re-formed by heat - thermoplastic under heat will becomple soft and pliable,
and thus can be welded).
BUT - successful welds need same-type plastic welding rod, so PVC rods will
not work (seal) polythene or polyurethane ponds (and vice-versa), for
example - and there's more than 3 thermoplastics!

If in doubt - google or contact the pond mfr (who should advise not only the
plastic it's made from, but also the repair technique)

Hope this helps

"john" wrote in message
oups.com...
Someone recently gave me a small pre-formed pond, but it has three 1-2
inch cracks. I was wondering what the best way to patch them would be.
Perhaps a bicycle tire patch, or silicone? Thanks.