Originally Posted by ksanders
No it does not make you "cutting corners cheap".
Personally I like to use All Weather Wood as backing blocks. I generally have part of a sheet lying around the shop. The stuff is indestructable.
Part of why I ask what the material is is because I think backing blocks are supposed to be a material with lots of compression strength. If the material is Azek which is a synthetic trim material, and is also used as decking material, I'm pretty sure it don't have great compression strength. I guess part of the problem is that I'm not sure what you mean by a butt block or a backing block. It's backing blocks for thruhull that I seem to recall need to have high compression strength.
Or better put, I'd look into the compression strength of any alternate material to be sure it's suitable for the job. I think D'Antonio has an article on this on his site. He seems to have an article for just about everything.
But to your original question, no, I think it makes you smart if you can find suitable, alternate materials. I've used Azek in many places as stand offs, as mounting blocks, spacers, etc. For the right applications it's great stuff.
One place I have been particularly happy using it is for a transom skimmer transducer mounting block. My tender has an aluminum hull, and thru bolting with stainless is a really bad formula with AL. So I used 5200 to glue a block of Azek to the transom, then screwed the transducer mount to the block with stainless screws. There is no stainless to AL contact, and I can adjust or remount the transducer many times before the block looks too much like swiss cheese. I'm gearing up to do the same thing again for a transom mounted VHF antenna.