preparing interior/exterior glassed joints for painting

[acreew]

Now that I am nearing the painting process.....the ridges left by the seams of the fiberglass tape (paricularly where they overlap) are concerning. I have sanded a LOT, but have sanded through the glass several times and I am afraid I am just making more work for myself (small holes to fill).

 

Any tips on how to smooth glassed joins so that the ridges are less prominent to begin with and secondly how to best smooth the cured joins so that it is not obvious when painted?

 

Thank you in advance.

 

Will

[Alan Stewart]

Typically on the outside of the hull a "putty" mixture of micro-spheres (white powdery stuff) and epoxy is used to smooth the transition from tape to plywood. This is best done with a large putty knife (about 6" is good) pulled perpendicular to the fiberglass tape with one edge resting on about the middle of the tape. The idea is to fill in a thin wedge of filler that eases the transition into the rest of the hull. Where there is overlapping glass tape the filling is basically the same but it may require more filler over a larger area. Sanding down the highest of the high spots in the fiberglass should be done before using any filler. 

 

Note that it is impossible to do this in one step with no pinholes or voids and a second or sometimes even third puttying will need to take place to finish the job. Sanding the micro spheres is very easy. This process can also be done with cabosil or B and B "special blend" but sanding will be more difficult and I wouldn't recommend it. Micro spheres are also much lighter than cabosil. 

 

 I usually don't worry about the edge being visible on the interior as long as it is smooth and there are no pinholes. 

[Anders]

Hi Will,

 

It's easy to shave down the tape edge ridges using many light strokes with a sharp scraper. Hardware stores sell nice ones with replaceable carbide blades that last well. Ordinary flexible steel cabinet scrapers work fine too, and can be flexed into a convex (or concave, I suppose) scraping profile that is nice for shaving down small areas. I have also used the replaceable blades for ordinary utility knives for scraping epoxy. For smoothing concave fillets, I have read that filing sharp corners on a round washer is also effective. It apparently is either held in the fingers, or gripped with a Vise-grips, but I've yet to try that.

 

I like to scrape epoxy rather than sand it, whenever I can. It seems to go faster, and creates zero dust. I do a light sanding after scraping, and then apply the microballoon fairing mixture, as Alan Stewart has described.

 

Anders 

[gizmo]

This sounds like it should have read "This is best done with a large putty knife pulled parallel to the tape"? 

 

Otherwise thanks for the info, it helps me with a project I'm working on.

[Joe Anderson]

I use a cabinet scraper or something similar to knock down most of the ridge on the side of the tape before using filler as Alan suggests. If you hit the tape before it is fully hardened it is a little easier. Sanding through or nearly through the tape can weaken it considerably.

[Fishman38]

Alan can the two or three steps you describe be done without waiting for complete curing between the steps? 

[Alan Stewart]

Sure you can. Work cleanly so you don't have spikes or lumps otherwise the second coat will bump over the bumps and leave more putty behind than you want.

My preferred mix for fairing is 90% microspheres and then a splash of cabosil to stiffen it up. As you add microspheres the mix tends to be runny and then all of the sudden it's too dry. A bit of cabosil kills the runny ness but doesn't dry the putty out so it sticks well. Also microspheres will easily double the volume of epoxy you start with so do a small batch until you get a handle on mix size.

[Howard]

My favorite way to eliminate the bump of the selvage edge is to wait until the epoxy is starting to stiffen up, then take a sharp utility knife and run down the inside edge of the tape bump and simply snip it off. You don't loose much tape and you are left with a clean sharp edge that is easier to fair in.

 

I've only used the microballons, which is the brownish purple looking stuff. Are the white microspheres better?

[Hirilonde]

I've only used the microballons, which is the brownish purple looking stuff. Are the white microspheres better?

/em flips a coin and it comes up heads.

 

So much of epoxy work can be made easier if you can devote a good bit of time at once and do stuff within the cure window.  Be it hot coating, trimming, fairing or what ever.  The best thing it can reduce is sanding.  And I will get up in the middle of the night to do something if it means less sanding.

[mattp]

<snip> And I will get up in the middle of the night to do something if it means less sanding.

Amen to that.

[LennieG]

Great info Alan and Dave and all. Thanks for sharing. This was all very timely for me as I am flipping and starting this process in next few days on my CS 17. I don't want to hijack the thread but allow me to ask a question which I believe is relevant for this stage. I am thinking thru the next stage and starting to think about paint choices. The boat building books I am reading all talk about bottom paints being necessary. All the CS's I have seen don't look like they have a "bottom " paint or maybe they do and it is white. My question:

Is a bottom paint necessary on a boat that is almost exclusively trailered?

Thanks

[Alan Stewart]

Good question. No a bottom paint is not necessary for a boat that lives on it's trailer. My CS17 is painted with the same paint top and bottom. I added some graphite powder to the white gallon to make grey for the bottom. I used Amercoat PPG brand 2 part Poly. It's a bit thicker than Awlgrip and not quite as glossy but just as UV resistant (so far) and is shown itself to be a good hard finish. Thinning with Awlgrip's slow brushing reducer worked very well for me. If you prefer Awlgrip, I have been pleased to find that you can buy base, converter and reducers on Amazon.com at very good prices. I have ordered Awlgrip products from the sellers "Atlantic boat supply" and "Merritt Supply" through Amazon and had no issues. 

 

Another option for smaller vessels is to leave the bottom painted with just epoxy primer such as Alwgrip 545. This way the bottom is all epoxy and scratches can be easily repaired and the bottom can be sanded very very smooth over and over again over the years. Especially good I would imagine for someone racing their boat. I know for a fact that Graham's EC-22 bottom is 545 primer for this reason. 

[Hirilonde]

I am surprised that no one here, at least no one I noticed is using System III 2 part WR-LPU.  It has come a long way.  The jury is still out as to whether it is as durable as the solvent based LPUs.  But considering the health and environmental issues, particularly concerning the isocyanates, I would think more would have tried it.  It isn't as glossy, but I actually like semi-gloss better on boats.

 

I really like 545.  It sands out beautifully smooth and quite easily if done within the cure window and becomes very hard after that.  Does it have enough UV resistance Alan?  Or because it is on the bottom it gets so little exposure it isn't an issue?  Graham stores his boat in a shed, so I don't think it is good evidence of this.

[Docpal]

Dave,

 Just before the Winter set in I used it to get a few coats on Petunia (CS#204). I've worked with many  topcoat systems before but none were as easy, or pleasant to deal with as the System 3 stuff. I HAD to go water based as my build has largely been done in the "stealth mode" due to local community restrictions on keeping a boat on the premises. Therefore most of my work has been done without the use of power tools, and using conventional marine paints would have brought out the blood hounds in most of my neighbors.

The primer and paint  both rolled on SO well ( white 4" "finish rollers" from Lowe's....) that most people who have seen the hull think the finish was sprayed. I used their application schedule of adding the catalyst to the final coats, and while she's still a few months from being wet, she can handle a lot of banging around in the garage without dinging, or scuffing . Time will tell how well the paint holds up in the sunshine, but since we haven't had much of that I'll have to wait until this Summer for real feedback.

[Joe Anderson]

I used System 3 WR-LPU on the interior of my cabin. I think it looks good. I had to be careful or it would peel up when I pulled the tape.

 

Concerning bottom paint it is softer generally than other top coats. I do not use bottom paint on my boat.  I have had my boat in the water up to a week without much growth. Much over a week and the barnacles start. I am sure this varies with a host of factors, salinity, temperature.

[LennieG]

Thanks for all the great information. It is really appreciated!

[Charlie Jones]

I  Graham stores his boat in a shed, so I don't think it is good evidence of this.

 

Yep, Graham stores Skimmer in a shed. And hurricane Irene tilted her up so the bottom was VERY easy to reach