Thoughts on using resin

by Simon Harris

Introduction

I have prepared this text as a preamble to the stage-by-stage painting guide, but I wonder if it wouldn’t be of some use in it’s own right. Resin dust is vile, unpleasant and potentially fatal stuff, unless you have serious respirator style dust masks, you should endeavour to sand wet wherever possible.


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Clean up – most resin casts fall into two types – fully encapsulated and pan cake moulded.

1: Pan cake mould is the simplest form of mould whereby with a piece such as a base the resin is poured in through what will end up as the bottom face of the finished piece. If you need to true up this is best achieved by sanding in a circular motion on a sheet of medium wet & dry paper taped down to a true flat surface. A mirror is ideal.
When sanding try and even out the clock wise and anti clock wise rotations and turn the piece by 90 degrees every half dozen circular motions. No matter how hard you try to press evenly you will always exert more force through your thumb and fore finger and by rotating the piece in your hand you help to balance the rate of wear so that hopefully the whole piece is reduced constantly.
Although you should always check it is really quite unusual to find anything by way of additional mould lines on a casting created by this type of mould.

2: Fully encapsulated – This means that the original master was effectively buried within the mould and unless it is a very simple mould-friendly shape, there will be a variety of flash marks where the mould will have been cut around the pouring throat to allow the piece to be de-moulded.
Because of the volume of material involved you need have less concerns during clean up, usually using a combination of scalpel/modelling knife and fine files. Resin is quite soft so proceed gently; a fine scotch-brite/scouring pad can be useful in restoring the surface texture.


Mould plugs, yes they are a pain in the butt but they are not put there for spite. The quality of the resin castings you receive have a great deal to do with the release of incorporated and entrapped air. Incorporated air, is bubbles within the resin in it’s liquid form, due to vigorous mixing, shaking etc..... this shows itself as an 'aero-bar' effect lying underneath what appears to be a good solid surface.
Entrapped air is air within the mould cavity, which has not been fully displaced by the resin fill. This shows itself as tiny, shiny hollow spheres particularly on sharp edges and also on what ends up as the surfaces upper most when the mould is filled. A vacuum tank goes a long way toward sorting both problems. Because of the speed with which the resin gains viscosity and coagulates, as the first part of a setting process, the size of the pour throat directly relates to how successfully the combined air content is removed from the pour. As with so much in life it is a direct compromise, the caster wants the biggest throat he can get a way with, the model maker wants the smallest possible.


Surface imperfections – No matter how hard we strive you may unfortunately end up with castings that require a certain amount of correction. Fine air bubbles are best dealt with a small dab of thick Cyanoacrylate (superglue) and a squirt of kicker. It is important that you clean up fairly rapidly as the hardness of the set C.A. will increase with time. As with any filling operation one of the most important criteria in the selection of the correct material for the job is the relative hardness of the filler compared to the substrate. This wants to be as close as possible. A filler that’s set substantially harder than the surrounding material will likely cause you more difficulties in the clean up that it solves.
For any other filling on resin components I am a big fan of car body filler, what you are looking for is a high styrene content and will go under the term 'Easy Sand'. The actual material is polyester car body filler and it is colloquially know as 'Red Dog'. I love it. You can fill with it, you can run it out as sheets to make masonry, you can make buildings out of it, you can make trees out of it and apparently you can even mend car bodies with it, although that does seem a dreadful waste of a valuable resource.


Having successfully found and rectified any blemishes, wash with soapy water, rinse thoroughly and leave to dry. One of the pit falls for the unwary, yes I found out the hard way, is the lack of opacity with most common resins, particularly in fine section. In practical terms this means that a thin prime of white and a light paint job may well leave you with a piece that is semi-transparent when held up to the light.
I learnt this one with some 12th scale rugged oak planking, which looked fine right until the time it got backlit for some photography. Boy, was I annoyed. Mercifully the solution is both simple and straightforward......prime lightly with black cellulose before applying a standard white primer. Neither coat needs to be particularly full, but if possible always use a spray can or airbrush, this builds up the paint fairly evenly onto all surfaces, brush priming can tend to fill incised detail.
Right that gets our pieces up to painting stage.

watch this space!


Simon