I thought it about time that I explained the use of the common filler powers that are used with epoxy. Basically epoxy consists of two parts, a resin and a hardener which when mixed together in the correct ratio can be used as a coating and a resin for such things as laminating fiber glass parts together using a fiber glass fabric. What makes epoxy especially useful to boat builders and repairers is that the resin can be used with fillers and fairing thickeners to modify the mix thus extending the uses of the resin far beyond just gluing and laminating.
There is a vast array of powders out there that can be added to epoxy and it can be little confusing at first to know what to pick and what can be used for which application. For general gluing applications and for the majority of gap filling requirements where more than one component will attach to another colloidal silica is a good bet. Off white in color it mixes in fairly easily and will bridge large gaps with no loss of strength. On the down side it is hard to sand once cured also the off white color will not suit gluing applications where the interior is finished bright such as a varnished trim or glued lapstrake boat made from Okume or mahogany plywood. In this instance filleting blend (pronounced fill-et) can be used which has a pleasing brown color and once varnished blends in beautifully with the wood. Not quite perhaps as strong as silica it does smooth out well and can be sanded if the need arises a little more easily. This is sold by Gougeon brothers as the west system 405 powder. For filling large surface areas and smoothing out a less than perfect profile say, then you need something that is going to feather out and sand easily then something like the microlight 410 is the way to go. Although a great filler for surface imperfections it has a lot less structural strength than any other filler powder so it is not recommended for anywhere where it is going to be subjected to loads.
In the photo above I have also included another couple of fillers that I use regularly, both from Interlux. They only offer two filler products, a glue powder which as far as I am able to tell is a silica but whiter in color and filler powder which is also white and although slightly easier to sand than the glue powder is still pretty hard and not something that I would recommend for filling large surface areas.
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