Final finish on new fiberglass - Gelcoat vs paint

Currently renovating 12 ft f/g o/b dinghy. Thwart seat, and aft transom compartments as built were formed from luan or similar with f/g over, and expanded polystyrene block flotation blocks inside. In the past, water got trapped in these cavities, and the luan rotted! I have cut out the thwart box/seat etc and constructed new one from 1/4" marine ply, well-coated with unwaxed laminating resin, then two layers of CSM with unwaxed.
Cavities are not yet closed off "watertight" so I have a couple of questions:
Questions: (1) Should I leave the old foam flotation out, and install access hatch so any condensation water can be drained?
(2) If foam is recommended - should I re-use the old - or use 2-part in small doses till filled, then put top (seat) on and glass in?
(3) Once wrapped in two layers of CSM with unwaxed laminating resin - should I finish with gelcoat or paint? What is best paint (latex, alkyd etc?)
Would appreciate best current thought on this since I am "newbie" on this (Only small repairs in past) Thanks.

 

Welcome to the Forum.

I've just done something similar as a project for someone. I ditched all the 2 part poly foam and cut up ceiling tiles in EPS, better to put in drain holes and hatches or breathers IMHO. Important that the new tanks can drain properly and air. I use cork bungs as they are cheap and light and easily replaced. I just don't see the point of using another water trap ie internal 'foam' or EPS block inside a sealed tank. Make sure you paint inside the tank except glue faces before putting the lid on...

As I did not want to bill the owner for several hatches, in this case, I left the option of retro fitting them. the important thing is the drain at the bottom.

I'd paint rather than gel on this type of thing. I used One Pack Poly as this boat was cheap and cheerful and someone had already used household alkyd based gloss inside her. Gel is cheap but very hard to get right on internal surfaces, outside maybe but sanding convex smooth shapes is relatively easy compared to the nooks and crannies internally. I also used a One Pack undercoat to fill the pores of new wood and seal exposed glass etc. This helps fill any little air pin pricks in the glass/resin as well giving a better top coat.

You may find 6mm (1/4") a little thin as a seat or thwart base. 9mm (3/8") is better, in fact I upped to 12mm with additional braces. Has to take someone wrestling with an outboard or jumping in after a few beers...

 

Thanks SukiSolo for the very practical advice! Good point re foams = trouble.

North American Coast Guard regulations call for dinghies with power over 2HP to have flotation compartments sufficient to support boat + occupants + motor + tank + battery according to a formula - and the flotation space MAY NOT BE INTEGRAL WITH THE BOAT. Strapped-in fenders, or foam etc are OK - but NOT enclosed cavities with just air in them!

Right now I am leaning towards paint on the fiberglass-coated wood of the under-thwart cavities, as you suggested - then somehow securing used plastic pop or milk bottles (with plastic screw-caps) in the space before closing off the "seat" panel. Air in the enclosed space sooner or later gives rise to condensation - so a removable plug is good to allow the space to be "aired" or drained periodically.

Re paint - my fiberglass store told me today that painting onto un-waxed (ie tacky) fiberglass mat or cloth is "iffy" as some paints will not bond? He recommended a final coat of waxed resin or gelcoat, then light sanding before applying ANY paint?

Would be interested in any other views/input from the group??

 

The USCG floatation recommendations are for manufactures, so unless you plan on getting a MIC and building a bunch of boats, you can handle the floatation issue as you like.

 

Interesting PAR, does that include RIBs'? Or maybe they put the flotation in the rigid part?

The main thinking in Europe is unless you have a single buoyancy chamber and have enough buoyancy with one 'tank' pierced you should be OK. After all if you split one 'tank' and all the bottles, poly beads etc floated out..... hence massive eps blocks and similar are sometimes used.
Like compartmentalised RIB chambers, it seems prudent to have enough chambers to ensure flotation on a rigid hull, if one chamber is pierced. Glass is SG 2.5 and poly resin 1.4 ish, if I remember correctly. Epoxy a tad heavier. Should allow quick weight calculation with approx 40-50% resin saturation on the build.

Never had much trouble painting unwaxed resin, it should cure pretty hard and pretty quickly. I do like to abrade though, and you can use a scourer 3M pad or degreased wire wool for awkward curves and edges to retain resin thickness. If you have pretty simple surfaces the gelcoat option is viable, just harder to get as flat and even. Mind you I don't use household paint, only marine or automotive formulations.....

 

Final finish on new fiberglass

Hi PAR: Thanks for the USCG clarification. I guess PET bottles or foam may still be better than an empty compartment since they would reduce the "free" air and potential condensation in the space? (Unclear why USCG won't accept floatation INTEGRAL with the boat?)

Re paint to use on "interior" hull fiberglass:
I would really appreciate help from the group on
(1) choice of paint? one part polyurethane? oil modified alkyd (Rustoleum)?, latex? etc I would prefer roll & tip-on matt/satin finish in gray.
(2) whether paint "clings" better to unwaxed or waxed final surface?
Attached three pics: (1) as found (2) old thwarts removed and new in place (3) current (not glassed)

 

You can't gelcoat or paint on a wax (styrene monomer) filled surface resin. The way the wax works is to float to the surface and create a seal from the air to let the gelcoat cure. If you abrade the wax off afterwards you can re gel or paint. Better to have a fully wax free surface to paint, no contamination, but it will still need some abrasion.

You can apply quite a few paint types but my personal inclination is One pack poly as a value for life expectancy and durability. Some older style alkyd based paints will also work, but I would not use much of the newer water based acrylic household stuff. Traditional ie older marine varnishes and paints were and are alkyd based, but a lot of so called yacht varnish stuff in UK in equivalent places to your Home Depot are rubbish and not worthy of the name. Some of the high end external alkyd household paints may be OK. I have seen some mixed with polyester resin (with hardener) before now.... and applied very very fast to old glass hulls.....

I've used 2k paints fine, epoxy primers, acrylic/polyurethane primers and 2k top coats no problem on glass or gelcoat. In fact some builders finish their boats this way. Just more expensive, than the other options. If you can keep the boat dry when not in use the alkyd based stuff may be fine, if not then a one pack will be superior. All of them will roll fine, you should not need to tip if you get the roll right. My preference is a 4" or cut down to 2" roller, hardish foam, and work in an organised way. Do one section say a foot to foot and a half get it really sweet and start the next section. Just as you do, quickly go back and gently roll the prior bit to pop any air bubbles remaining. Timing this second action is the key to getting a good surface with no bubbles. A quick wave with a hot air gun ie hair dryer, paint stripper gun will pop any remaining bubbles. You should get a pretty fair surface, though good brush technique will be mighty close....

Oddly enough I just used the Toplac (International) grey on the last beast as well as the white undercoat. You can also add the fine non slip granules for anything on the floor too. These are mostly ground inert pumice. The overcoat window on One packs is usually around three days depending on temperature so you get quite a useful working window. They generally cure a bit harder than alkyds, but not as brittle as 2k stuff.

 

Thanks again SukiSolo for the "voice of experience" - much appreciated!
Hempel noted that 2-pack poly is less brittle than 1-pack. They also steered me away from bilge paint - recommended Hempathane (2-pack Poly) but it is a 7:1 mix and lots of nasties (isocyanate) so said I should buy a 3M carbon respirator for Organics.
Rustoleum Marine Primer and Topsides is pricey, and the Porch & Floor Enamel is not offered in Canada - so ended up looking at Armorcoat Acrylic Latex Porch Enamel (water-based). Will buy a quart and try it soon . . . on some test fiberglass.
BTW Glidden, Pittsburg, Valspar, Behr, Rustoleum etc all stated in writing that their Porch paints should not be used for this!
Remember this is for the inside of this 12 ft open hull - NOT the water-side.

 

hi jonboy .. i'm an autobody painter ..

here's my 2c on the paint subject ..

use single stage urethane .. it's available at any ppg paint store and is for automotive use .. the best bang for your buck is MTK formulation .. it's just paint then you add hardner and reduce it a bit for good flo .. it runs only about 40$ a quart and there are 1,000 color choices ..

sticks well and is compatable with almost everything .. i use it on all my trucks and recently painted my own boat with it ..

good thing is also when you get a ding or scratch you can repair the spot and scuff the surrounding pain t .. tape the box with a curl on the edge so no hard line .. paint the repair .. then untape .. wetsand to blend and buff .. and it looks good as new .. i expect to repair my boat frequently as we have a rocky lake here and i 4x4 my trucks .. so ease in repairablity, cost, and appearance are my primary concerns ..

if you use it .. go for the fast hardner even in warm weather ..

as for adhesion .. use "bulldog adhesion promoter" it's 25dollars in a spray paint can .. but one lasts along time .. i use it to paint on non cooperative surfaces and when i'm just not sure .. i use it to spray plastic bumpers and it doesn't chip off .. recently used it for the bare fiberglass surface of my hull i shot a few days ago .. just a mist coat and spray as normal 5-10 minutes later ..

that'll keep er' on there .. good luck .. i hope it turns out great .. btw u can see the paint i sprayed on my boat in the thead entitled .. about first plug .. and have a conern .. later gator