best material to be used when fairing?

Hi Guys,

At present when we do large gelcoat repairs on joins etc we have been using cabosil powder mixed with gelcoat as the final filler after csm before gel spraying.

Its as hard as nuts and a ***** to sand. Can anyone out there with experience of this type of gelcoat repair possibly recommend a suitable filler powder for use with polyester gelcoat that is very strong , lightweight and easy to sand?

Thanks.
Neville. Scotland

 

q-cells or microballoons or west microlight powder, or any other brand of "low density" powder modifier... silica type powders and microfibres etc create a "high density" mixture for structural bonding type applications. Alot of people use "a little" cabosil in their low density fairing mixtures for its thixotropic quality - its helps it non-sag - but if you use too much it gets harder to sand...

 

Nevilled, you would like to mix in some talcum or ATH, which is ultra cheap. Most polyester suppliers should be able to help you with that. (it is used in polyester to reduce cost, shrinkage, and improve fire retardency).

Disadvantage of ATH is reduced water resistance.

 

My main concern is strength. We can in areas put the filler on 3 - 5 mm thick when fairing complex radiusses on wheelhouse joins.

Cabosil has proved its strength to us, but its just so time consuming to sand and therefore requires a bit more skill.

I will get a bag of Q cell tomorrow. I have seen this before but have been worried about changing.

I suppose some simple mechanical tests would help.

Thanks. Neville

 

If you want strength, you'll use predominately "fibrous" materials as your filler. Wood flour, silica, milled fibers, cotton flock, etc. all are very fibrous and offer lots of strength to the filler. These materials are difficult to sand, understandably as they're intended to offer stiffness and strength. You can ease sanding effort slight with talc and other non-fibrous materials, such as q-cells, balloons and spheres. These have much less surface area for the goo to stick to and typically are hollow, so very light and easy to sand. They offer limited (if any) strength in structural applications.

Generally, fillers fall into three categories: heavy structural, light structural and cosmetic. If you need a heavy structural fillet (for example) leave it shallow and over coat with a cosmetic filler after the fillet is shaped. This will offer something to sand, but still has the strength and stiffness requirements you need. This can be done in one operation, while the goo is still "green". Make the structural application, again just shy of where you want the finished surface, clean up globs, drips and ooze out, then apply the cosmetic compound over it, neatly to save some finishing effort.

 

although im a year away from wholesale fairing, this is something i still need to get my head around. it seems to me the downside of mix-it-yourself fairing is the imperative of using the exact same ratios with every batch regardless of meteorology otherwise you end up with different densities and sanding rates, making it hard or impossible to true-out your surface. is this a reality or am i over thinking it?
and is there a ready-mix thick film fairing compound for frp above and below the waterline?

 

Yes, you can get polyester spraying compounds. They will be consistent.

 

thanks gonzo but im trying to stay away from poly based fairing compound as my modifications are epoxy/biax. are there pre-mixed epoxy based fairing compounds that dont break the bank? and another thought... i buy epoxy by the 5gal bucket. what if i set aside a bucket full of resin ONLY that has been pre-mixed with baloons and some cabosil, and only add hardener to small batches as i need it ? seems this will ensure uniform density throughout the whole bucket full even though its many different batches over time/temperatures. anybody tried it?

 

PAR : Maybe i could try experimenting with Q cells mixed with milled fibres, I suppose though there is probably no magic material thats light , real easy to sand and structurally very good. Just a lot of hard work is required.........

 

Once you get some experience mixing filler combinations, you'll have little trouble making fillers to suit whatever need you might have. You can sometimes use light weight materials in a structural fillet, but typically these fillets need to be 3 times as large (minimum) to offer sufficient strength and stiffness, which uses resin up a lot faster.

Most premixes (available in epoxy too) work well, but are costly compared to mixing yourself. QuickFair from System Three is a good product, but again the cost. You might want to pick up a small amount of this stuff, just to get an idea of the stiffness and consistency. This will help in developing your own fairing compound mixtures.

 

I have found that a 50/50 blend of silica and Qcell makes a filler that is easily sanded, thixotropic and reasonably strong mix, i havnt used it with gelcoat though.

Steve.

 

Why are you having to use lots filler if its just a join?? why not use more glass and build it up then just use auto filler ,sand then gel coat over the top like a million other people do !!!
Just remember these basics and do your own mixes !
fumed silica (areosil)is just to thicken resin or gelcoat ,
Q cells , Microbaloons etc etc is for fairing and makes everything easy to sand . Has not much guts to it .
talc make thing hard to sand
Micro fibres is to help think build up not crack so easy !!.
Auto bog is easy to use and quick and makes a nice job !!

 

Tunnels, one problem is that this is a stepped join, its where an outwardly pointing wheelhouse flange is bolted then glassed over on to the deck. So its a trick join that takes a damn good finisher about 4 days flat out to do. Ultimately the wheelhouse mould will get replaced with an inward return which will cover the bolts and mean no external glassing and finishing. One of these sod of a jobs that if the mould tooling had been better thought out in the first place then we would save alot of time in production.

 

See the problem. (see it all the time, actually...)

 

Yup, a few more hours spent at the beginning would have saved alot. Now we are in production its far more difficult to get the time to retool....