CPES Application Questions

[Capt. Mike]

I found that our CT42 mahogony bowsprit had a lot of rot, so I decided to replace it. I glued five clear vertical grain fir 2x8 boards side by side using West System epoxy and then shaped the result with a chain saw. Because the bowsprit will be exposed to the elements and will ocasionally dip into the sea, properly sealing it against moisture is important. I ordered CPES after reading about it in other threads on this forum, but I've never used it before. My plan is to coat the entire bowsprit with CPES. My questions are how many coats of CPES and should I encapsulate the bowsprit with epoxy over the CPES?

Thanks,
Capt. Mike

[PAR]

CPES is epoxy, though it's a thinner formulation, designed to remain flexible, mimicking the wood abilities. 2x8" material is very thick stock to laminate successfully, most recommend no thicker then 1" laminations, so keeping this stable is very important. Apply CPES to bare wood only. It's of little value on anything else. Keep applying coats until the wood will suck no more of it in. The best protection would be paint, though you can use a clear finish (urethane or varnish) if desired. Straight epoxy over the CPES will harden the surface a bit, but not offer much abrasion resistance without fabric (cloth). Epoxy over the CPES will not offer much help in keeping the sprit stable, only good care and maintenance will. Clear finishes require more maintenance then paint. The big enemies to a sprit will be UV break down under a clear finish, repeated flexing delaminating the 2x8's and moisture ingress. Keeping the coatings (paint/varnish and CPES) intact (repairing dings and scratches quickly) will help keep the wood healthy. Properly tensioned stays will minimize undue flexing which could break out the glue lines in the laminate.

For CPES or epoxy of any type to work, every surface must be coated. Every screw or fastener hole, every cutout or notch, every side, etc. It literally needs to be embalmed in goo. Pay special attention to the end grain, where it will require more coats to seal up.

[LP]

PAR,

A little confused about why 2X material is less desirable than say 1X material. Especially in a straight member. Generally, laminating involves thinner material for the sake of bendability. In theory, the epoxy joint is stronger than the material it is bonding, provided the material is compatible. i.e. fir(?) Perhaps thinner sections provides for a more random grain matrix and thus a reduced internal stress concentration.

I've been known to sandwich a layer of glass between laminations. Not necessarily the best location for maximizing cross-sectional moment of inertia, but useful in building some cross-grain strength into a stuctural member.

Lastly, would CPES work as a "base" for better adhesion of an epoxy/glass coating? i.e. Since CPES penetrates and forms a substrate for the epoxy to bond to in addition to the material itself.

[PAR]

Once you go much past an inch in laminate thickness, there's a likely chance that dimensional stability can change, as a result of too much mass in cross section. Douglas fur has a lot of internal stresses (some release and form surface checks), which will compound the issue.

LP, you're correct that the spring back issue, common in thicker laminates, as well as the bend ability of the stock, in thinner laminates aren't problems in a straight beam.

CPES is a great raw wood coating, but my tests show it doesn't have the solid grip that straight epoxy does. It's flexible and can sheer in high loads, well before a properly prepared regular epoxy bond will. In areas I know will have high loads, I heat the piece that will receive the epoxy (120 degrees) then apply straight epoxy. It penetrates, very deeply and cures hard, not soft like CPES. I still coat raw wood with a penetrating epoxy, but do grind away areas that need a structural bond.

[Capt. Mike]

Thanks for the reply. If I understand correctly, I sould apply CPES until the wood will not absorb more and then paint over with a polyurethane paint. It was not my intention to leave the sprit bright. The final coats will be white paint, but I wondered if epoxy should go between the CPES and the paint to build up a thick hard layer to better protect the spar from scratches and dings.

Thanks again,
Capt. Mike

[PAR]

Epoxy alone offers little abrasion protection. Cloth set in epoxy provides very good protection and is commonly done on wood surfaces for this reason.