The interlaced fibers of the Alsi 20 Fiberglass Filler provide a superior finish and quality to the repair in less than 20 minutes.

Most Fiberglass Fillers shatter on impact and will leave marks like spiderwebs on surfaces.  Alsi 20 is a superior Fiberglass Filler that will not leave any such telltale signs and will not shrink or absorb moisture.  Alsi 20 Fiberglass filler is perfect for repairing Fiberglass on cars, in boats, Sea Doo’s, Ski Doo’s etc., as well as many other polyester materials.  Used by repair specialists all over the world for a strong, long lasting and beautiful finish.


  • Extremely strong, yet fast working
  • Easy to use
  • Flexible
  • Will not shrink
  • Will not absorb moisture
  • Superior repairs
  • This polyester resin was especially designed to work with Fiberglass and polyesters
  • Will not clog up sandpaper

Application instructions for Polyester – Fiberglass Filler

All polyester bodies.  Refrigeration tanks on trucks, cracks or dents in Bumpers or hatchbacks, for filling up a crack in a hull, on frames of snowmobiles, small pleasure water craft, large boats, motorcycles, etc.

Application Procedure:
Make sure area to be repaired is clean, dry and rough.  Make a V incision on area to be repaired to give the area some depth and volume so Alsi 20 can bond easily.
Mix contents in can thoroughly.  Take out a little Alsi 20 from can and place it on your clean work mat (clean smooth flat metal surface or other clean flat mixing surface).  Add approximately 2 % hardener to the contents outside the can and mix thoroughly.  Apply generously in layers over area that is to be repaired.  If you find that it is taking too long for Alsi 20 to harden you can add some more hardener the next time as this will speed up the hardening process.  Normally Alsi 20 takes 40 to 45 minutes to harden after which it can be sanded.

Alsi 20 Fiberglass filler starts to harden very quickly (3 to 4 minutes) so mix small quantities at a time.
Avoid inserting the mixing paddle into the Alsi 20 can after you have used it to mix it with the hardener.  This will contaminate and harden the contents in the can and make it unusable for future use.

Technical information for ALSI20

Q: Is Alsi 20 waterproof ?
A: Yes Alsi 20 is waterproof.  Alsi 20 will not absorb water or humidity & will not bubble or pop off like many other fillers.

Q: After I repair a crack or gouge on a Fiberglass or polyester surface with Alsi 20, how long will it stay repaired ?
A: It should stay repaired for at least 7 years.

Q: Is Alsi 20 hard to sand ?
A: Alsi 20 is harder to sand than Alsi 12 or Alsiflex, but still easy to work with.  Once you mix Alsi 20 with the activator, it will start hardening in 2 minutes and you can start sanding the surface in approximately 20 minutes.

Q: Can I repair aluminum with Alsi 20 ?
A: No, Alsi 20 is used to repair Fiberglass or polyester surfaces.  For aluminum surfaces use Alsi 12 or Alsiflex.

Q: Can I repair plastics with Alsi 20 ?
A: No.  Alsiplast is better for all plastic repairs.

Q: Is Alsi 20 flexible. ?
A: Alsi 20 has some flex in it so that it will not crack or break easily.

Q: Can I add a light polyester filler over Alsi 20 to finish the job?
A: Yes you can, however if you apply Alsi 20 correctly, you won’t need another filler.

Q: How long does it take for Alsi 20 to set before I can start to sand the area ?
A: It takes about 20 to 30 minutes for Alsi 20 to set.

Q: How much hardener do I mix with Alsi 20 ?
A: Add 2% to 5% of hardener to the total volume of Alsi 20 that you need.

Q: Can I prime directly over Alsi 20, or do I have to use another polyester putty over Alsi 20 before priming and painting my surface?
A: Yes, you can prime and paint directly over Alsi 20.

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