Next step after sandblasting?

[Resolution]

After sandblasting to remove years of anti-fouling debris, my hull has been left beautifully smooth - but the gelcoat has loads of small cavities, up to 1mm diameter. (No osmosis).
Does the team think I should fill & fair these first, or can I move straight on to painting with an epoxy primer?
ie Will the epoxy primer fill these sort of cavities and leave a smoothish finish?
Peter

 

[macnorton]

You should repair all marks before primer is used, if you flat the surface with wet and dry it can remove a lot of small blemishes.
It depends on the finish you want and how far you are prepared to go.

90% of repainting is the preparation.

 

[Bosun Higgs]

How often do other sailors see the underside of your hull? Personally I wouldnt bother filling. The finish of epoxy can be a bit eggshell like anyway. Certainly when I did my last boat, there was just me to do the work, no one to lay off and the finish was "golf ball". I'd swear it made the boat faster.

 

[Plevier]

Are your cavities air bubbles in the gel coat that have been blasted open? That's what I had, lots of them in patches.
They were too small to get filler into, the air couldn't escape. You try and press it in and it just exudes out again.
I went over them with a mini drill and a small spherical burr opening each one up into a little dimple, took just seconds for each, which I then filled with solvent free epoxy filler smoothed in with a plastic blade prior to epoxy primer all over. Didn't take long.

 

[sailorbaz]

Just had exactly the same with my boat - some areas very smooth but patches of pock marks 2-3 mm diameter here and there. There were more on the port side than on starboard.
I filled with International Epifill using a plastic spatula 1 1/2" wide and my son then faired it for me with 40 grit on one of those iron shaped sanders - it's now as smooth as a baby's bum.
Good luck with yours.
Sailorbaz

 

[rob2]

Certainly when I did my last boat, there was just me to do the work, no one to lay off and the finish was "golf ball". I'd swear it made the boat faster.

Interesting, I seem to recall that dolphins, amongst others, have a textured skin which is believed to hold a layer of water aginst the skin so that the separation zone when moving is water/water so less resistance. Can't remember the physics, but golf balls are dimpled so that their spin has more effect on their trajectory.

Rob.

 

[theguerns]

Its funny you say the boat was faster as I have a little story to tell you. For years I have designed Yachts for junk rig. One of the principals of the junk rig is the way the wind accelerates at the moment of hitting the battens, this is whats known as a turbilation effect. The wind is tripped which causes it to tumble and accelerate. I have had tests done in a wind tunnel which have shown as much as 6% increase in the wind speed between the Luff and the Leach. Sorry I am going on. A friend that races an X33 always had his anti fouling polished like a mirror and told me this was to make the yacht as fast as possible. I told him that having such a fine finish on the hull was actually making the boat slower because the boat was sticking to the laminar surface of the water. He listened to me and applied his anti fouling with a fluffy roller so that the finish was rippled. Yes you have guessed it the boat actually went 0.4 knot faster in the same wind and on the same point of sailing. The reason is quite simple with all the ripples on the anti fouling as you say the golf ball effect it breaks the laminar surface there fore breaking the clinging effect of the water and letting the boat move through the water faster. Another bonus is when you are motoring you use less fuel.

 

[laika]

The finish of epoxy can be a bit eggshell like anyway. Certainly when I did my last boat, there was just me to do the work, no one to lay off and the finish was "golf ball".

Sorry to show my ignorance but tell me more about this "laying off". I had a couple of goes at applying Blake's sfe 200 to my rudder with both foam and felt rollers. Couldn't get it to stay smooth and it ended up with a slightly ripply surface: minimised by doing ma.ny thin layers rather than fewer thicker ones but wonder how I could have got it perfect

 

[Hydrozoan]

I had a couple of goes at applying Blake's sfe 200 to my rudder with both foam and felt rollers. Couldn't get it to stay smooth and it ended up with a slightly ripply surface: minimised by doing ma.ny thin layers rather than fewer thicker ones but wonder how I could have got it perfect

Laying off is smoothing out the brushstrokes / roller marks, I believe. We did our hull and rudder with 2 coats of Blakes SFE200 and it was indeed a tiny bit dimpled (not 'ripply'), but I don't think a second person laying off after the roller would have made much (if any) difference, as it was pretty cheese fondue-ish stuff. But we were pleased with the finish then - and even more so now, many trouble-free years later. I'm intrigued that any dimples might have made it go faster, but a few years of antifoul on top will surely have filled them . I can be a bit of a perfectionist, but there are limits and I'm with Bosun Higgs' "Don't be too fussy" approach.

 

[Resolution]

Thanks guys for the helpful remarks. We are going to try to fill & fair, before moving onto the epoxy primer.
Can someone now fix the weather for me?