Sealing through deck holes in balsa sandwich

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ex-Gladys

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All,

Have made an offer on a "new to us" Colvic. It has one dog's breakfast of a so called ventilator and others that will need replacement. The "dog's breakfast" has no through deck tube, and the balsa is open. How should it be treated? Also when installing new vents, what is the recognised method? Does one just use sikaflex, or do you treat the "hole" first with epoxy or something?

Thanks in advance /forums/images/graemlins/smile.gif
 
Old trick ..... cut hole through deck and then .... >

Get a l...o....n....g nail or other stiff wire rod .... sharlpen the end and then bend it to form a L shape .... stick this in an electric drill ... prefer one with speed control !!

Now use this to flay the core between the grp skins back a reasonable amount ... in fact it doesn't have to be that far ... but enough to be able to back-fill with good mix of epoxy and filler .... in fact polyester is fine ... despite others who may SHOut EPOXY !!

The main point is to a) provide solid filling between the skins so that load can be carried by fitting and b) that the balsa core is sealed .....

not a hard job ---- just messy !!

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G'day Larry,

As the loading of a vent is light you can mix some epoxy resin with Micro-balloons or 'Q' Cells after removing about 10 mm of the Balsa core material.
Sand to a smooth finish after curing and coat with any paint that stop UV light.

For other heavier applications, block points, winches and so on that are attached with bolts, drill the hole oversize, and remove 15 to 20 mm of core, place tape on the bottom, after filling and curing (24 hrs) redrill to the required size. The filler here should be Micro-Fibres not balloons: just make sure you wipe any access material away while it's still wet or you will have a hard sanding session ahead of you.

Epoxy will bond to the Balsa core, standard resin will not bond to any timber long term, epoxy will also bond better to standard resin as it will chemically bond as well as mechanically, standard resin will only mechanically bond.

I hope this helps.

Avagoodweekend......
 
Resins ....

Sorry to be a pain again ....

again I quote model building .... Many model airplanes have foam cored wings that need jointing in centre. The joint is sealed well and then the joint wrapped with GRP bandage so that the GRP is out about an 2 - 3 cms overlap on to the wing itself. The resin usually used is Polyester, as it soaks into the bandage AND wood better than thicker epoxy. It certainly bonds well and rarely fails. The only thing is to make sure joint is well sealed to stop the resin meeting the foam - which it melts.

The core of this deck in the original post is Balsa .... which has the ability to soak up any thin resin and so increase its bond. Polyester or Epoxy.

Finally epoxy resin and Polyester are actually not the best of combinations ... and I question the 'chemical' bond of Epoxy to the Polyester surrounding the fitting .... as I understand - both in this instance would be Mechanical bond ....

But of course I could be wrong !!

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Epoxy v Polyester

This one comes up all the time. Experiments have shown that when bonding to cured GRP, Polyester produces at least as good a bond as Epoxy.

In this instance, as I understand it your hole is a big one and there is a risk of flexing which could crack a resin/filler mix and let water into the core. You need to add some strength to your filler so I would advise epoxy with micro-fibres which are bought as a lumpy white powder. This combination is used to join plywood panels by just applying a fillet to the join with no other reinforcement so it shouldn't crack in your application.

For use around bolt holes etc, Nigel has the best method. I use Fillite as the thickening agent and mix it just sloppy enough to flow into place and fill the voids.
 
One of the best fillers / strength agents ....

Devcon are a company that makes various resins ... polyester and epoxy. (Please no jokes about Devcon 4 - run for cover !!)

They produce a grade that is actually polyester with fibres already in the mix .... the fibres are reasonably long but the mix is easily worked. It comes with a separate hardener as with most resins ..... This stuff when mixed is one hell of a strong way of doing anything like this .... when in tin - it is light grey, once set it darkens ....

It may not be cheap - but it sure as hell does the job !!

One tip before bonding .... GRP being based on Polyester normally is better bonded to after a wipe with acetone ... which softens the surface of the item to bond to ... plus it cleans away any crud. If you want to bond to Gel coat - then it MUST be roughened seriously - as Gel Coat is 'like grease' to any resin and hard to bond to ... it needs roughing and scoring as well as the acetone treatment .... in fact I would go so far to say that normally I would remove gel-coat and bond to the laminate under !!

Don't forget that any resin does not like to be sanded !! It is hard, clogs your sander ... etc. So get it as smooth and as near to finished as you can before sanding !!

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Re: One of the best fillers / strength agents ....

A few points to add there. Gelcoat almost always has traces of the mould wax on the surface which must be removed. While i was building my boat I made up a test piece of moulded surface divided into 4 sections:

1. Untreated
2. Washed with acetone
3. Abraded with angle grinder & 24 grit disc
4. Abraded and washed

I then bonded a patch onto each section and after curing tried to tear off each patch. The result was that patches 1 & 2 tore off cleanly with little effort. Patches 3 & 4 could only be removed with great effort and the failure was within the laminate, not at the join.

Gel coat contains no reinforcement so a bond onto it will be weaker than if it is ground back to expose the underlying laminate.
 
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