Max, given the rubbery nature of the "set" urethane its not condusive to sanding and painting. A guy could probably scuff pad the repaired urethane area and shoot it with a dusting of paint.
The key to a good repair is to get a good etch into the plastic with the sandpaper or grinding disk. You will see how incredibly sticky and adhesive the product is soon after you chaulk-gun it on. Clamping or weighing down the repair will really help too.
Ask for SIKAFLEX. Novus and other glass shops use it in Fairbanks. I've been repairing things with this stuff for twenty years. Most guys throw away things that I've been able to bond and repair.
It works excellent for hinge material that is broken off of sled cowls...
Which version of sikaflex do you use? There seems to be 5. They are 291, 291-lot, 292, 295 and 296.
I came across this site and thread yesterday while searching for methods to repair Polyethylene Kayaks and found the information very valuable. I was especially interested in reading about Frank's Sikaflex 291 solution, but also wanted to add a few bits and pieces of my own which others may find to be of some assistance. The Sikaflex solution sounds like the most cost effective method of repair. I phoned Sikaflex Tech support (Aust) about a month ago but at the time was told they manufacture no product suitable for use on HDPE (High Density Polyethylene), but I'm also guessing that no one in their technical department has ever tried to use it to repair Kayaks, and it sounds like Frank has had quite a lot of successful experience in this area.
Please note, I have not tried any of the items or procedures below personally. At this stage I am still trying to gather every possible piece of information I can find before attempting to perform my own repair on a Polyethylene Kayak which has developed a slight hole on the base near the keel section.
Aside from the obvious plastic welding, here are a few other suggestions.
In terms of adhesives suitable for HDPE (High Density Polyethylene) - Loctite 401 in conjunction with the Loctite 770 Primer is probably the most promising adhesive for bonding HDPE patches, not certain however of its water resistant/proof properties - Loctite suggested using the adhesive for the patch followed with a suitable RTV silicone around the edge to form a waterproof barrier.
See http://tds.loctite.com/tds5/docs/NEW-CA401-EN.PDF and http://tds.loctite.com/tds5/docs/770-EN.PDF for technical datasheet. The product specs for 701 appear to suggest that 701 is quite resilient to water even after 5000 hours of exposure.
3M also make a product called Scotch Weld DP-8005 http://www.3m.com/intl/kr/img/adh/adhesives/dpseries/DP8005.pdf which will bond HDPE, however it breaks down when exposed to water for prolonged periods, further, you need to purchase a special dispenser which is very expensive (many times more than the actual adhesive).
Several days ago I came across a product that also sounds quite promising called "Patch 'n Go" http://www.moldingraphics.com/Foreign/1Italian/PNG.htm , essentially it's a form of semi solidified plastic compound which can be moulded and shaped to suite the crack/hole area and is then activated by use of a low burning flame causing it to weld to the surrounding plastic. Unfortunately I cannot locate a supplier/distributor here in Australia, however in your part of the world it appears to be readily available.
Apparently flame treating the surface of Polyethylene using the correct setting on a propane torch can also provide the benefit of changing the PE surface molecular structure enough to allow it to bond with other types of adhesives that may not traditionally bond to PE.
see: http://www.combust.com.au/plastic/polyol.htm & http://www.a-a-i.net/polyethylene.htm for detailed procedure and description. I am wondering whether the flame treating could be used in conjunction with the sanding suggested by Frank to further increase the bonding of the Sikaflex 291 to the plastic?
Some other useful links:
http://www.patchngo.com/ (note: this is not the same as the "Patch 'n Go" mentioned above.
Hope someone finds the above useful.