Tim Ballinger Posted January 23, 2020 Author Share Posted January 23, 2020 Well added the 3 layers of cloth and the final job looks ok. However when I started to apply the first layer it seemed as though the gel, by now a semi cured skin , was coming away from the mould as I stippled the cloth. Now I’ve never used coloured gel coat before and I am not sure I would have seen this with the clear gel coat. However I am nervous I either got the gel too thick or went with the cloth layer too soon . Anyone noticed this or had it happen to them? Guess I will find out on Saturday morning when I release the cowl from the mould. If it’s gone wrong then I’ll be on cowl no 3 ! 🤞 Edited By Tim Ballinger on 23/01/2020 17:33:19 Quote Link to comment Share on other sites More sharing options...
Colin Carpenter Posted January 23, 2020 Share Posted January 23, 2020 Tim if I remember correctly , sometimes polyester and epoxy don't mix ! Poly over epoxy was ok but not other way around 😥. Or perhaps the other way ? It's 15 years ago ! Normally the gel would go finger dry within an hour in my fan heated shed , even on days like today ! I used the same brand pigment you have there. You didn't need much. Colin Quote Link to comment Share on other sites More sharing options...
Tim Ballinger Posted January 23, 2020 Author Share Posted January 23, 2020 Colin , you are correct about the incompatibility but this poly gel is advertised as being specifically epoxy compatible so as to get the benefits of both compounds. It seemed ok on the first cowl out of the mould but just not sure it feels right this time but we will see in due course. Tim Quote Link to comment Share on other sites More sharing options...
Tim Ballinger Posted January 25, 2020 Author Share Posted January 25, 2020 Cowl no 2 now cured. Complete disaster! As I suspected I messed up and the gel coat had indeed come away from the mould when I had started to lay the cloth. I hate to show it but the result is below. It is now in the bin. So my enthusiasm for using coloured gel coat where I could see what I was doing was obviously part of my undoing. In an effort not to get it not too thin I brushed way too much on and it sagged . I saw the ripples on the inside of the mould but had assumed the mould face side would be ok. Wrong. Second error I think was starting to work the cloth layer too early in the gel coat cure. Guidance says best adhesion is to start when still tacky. Although my over thick coating clearly did not help I think I went too early and as I suspected pulled the still rubbery gel coat away from the mould. Despite stippling it all as flat as I could the result was inevitable. Great bond just looks like a frozen lake. Glad I have 1 decent cowl in the bank as it were. So hopefully with a lesson learnt I will try cowl no 3. Tim Quote Link to comment Share on other sites More sharing options...
Piers Bowlan Posted January 25, 2020 Share Posted January 25, 2020 I empathise with your disappointment Tim . Just a question. Why do you want to use a gel coat? I moulded a Sport 500 fuselage from the supplied acetate shells provided with the kit as a mould. After applying wax and release agent I used some epoxy mixed with some thickener, which I painted onto the mould. When dry I added the layers of glass cloth with more epoxy well stippled in. It seemed to work OK and would have been even better if I had taken a little more care to avoid one or two small bubbles forming in the epoxy/filler mix. I guess it is all down to practice. Quote Link to comment Share on other sites More sharing options...
Tim Ballinger Posted January 25, 2020 Author Share Posted January 25, 2020 Firstly, where is Levanter when you need him, he was a professional in composites I seem to remember and would give a better answer than me I am sure. You do not have to use a gel coat. However if you use a gel coat you have a thicker layer of resin protecting the laminate which is the important structural element and any damage to the fibre is bad news. Cosmetically this looks better and it is easier to achieve a high gloss finish. (I am beginning to get out of my depth without doing some reading but in using a polyester gel you get a more flexible finish than if you used an epoxy gel . If you want a good clean hard edge/surface than epoxy gel is I believe the way to go.) The bond between polyester and epoxy is not as easily achieved and even with special formulations will not work well after the polyester is fully cured. However if you have a polyester gel between the laminate and the mould , which probably has an epoxy resin surface you protect the mould if you penetrate the release agent or wax when laminating with your epoxy. Finally if you add pigment you can to some extent protect against UV damage. So as you suggest adding a gel coat for our purposes might seem overkill and not worth the effort, especially when it does not go well. To some extent having made a useable cowl (with clear gel coat) I am now just experimenting while trying to also make a spare. i have just tried again and made sure the gel coat application was not too thick and I let it cure for longer. I did not see the gel coat pull away from the mould this time but it was very difficult to get an even coat . It was almost like the effect you get in paint if you have mixed in the wrong thinners it just seems to separate out. i did use a different manufacturer for the pigment paste but it was sold as polyester compatible and seemed to mix ok. Sorry may have got carried away there but I have been thinking a lot about the whys and wherefor’s as you can probably imagine. Tim Quote Link to comment Share on other sites More sharing options...
Tim Ballinger Posted January 26, 2020 Author Share Posted January 26, 2020 Cowl no 3 : Not perfect but quite pleased with this one . Certainly useable just needs a few blemishes removing. It’s actually the same weight as cowl no 1 so some consistency. After the failure with cowl no 2 I was very sparing with the coloured gel coat thickness and you can see the brush marks coming through or rather the light coming through so certainly not the perfect solid colour. Really a bit academic as it will be primed and painted. As far this cowl goes that’s it now as there are two very useable cowls for Paul to play with on his 172 build project. Had I waited longer for the gel coat on cowl 2 to cure I would probably have had a good solid colour but it would have been at the expense of extra weight as that failed attempt came out at 42g. I’m still not sure why the coloured gel coat appears to be so difficult to brush on evenly. I am even wondering if the new brushes I used had something on them that caused the issue. I have one new brush left so I might give it a good clean first then try brushing a sample on something to check it out. If not that then perhaps sticking with the same manufacturer for paste and resin might be the answer. Anyway that’s it folks, mission accomplished. Tim Edited By Tim Ballinger on 26/01/2020 14:58:48 Edited By Tim Ballinger on 26/01/2020 15:06:01 Quote Link to comment Share on other sites More sharing options...
Martin McIntosh Posted January 26, 2020 Share Posted January 26, 2020 I have made very many cowls and other parts over the years but by a process of trial and mainly error have settled on this method: 5 coats of release wax each buffed after 20 mins, then a gel coat made from ABL Stevens or Bucks Composites 24hr resin plus enough of Bucks thixotropic agent to make the mixing paddle stand up in the pot. Brush on a thin layer then leave in the airing cupboard overnight to set. Another coat of the same, back in the cupboard but only until it starts to set then a layer of 25gm cloth. leave to set, then more resin left as above until it is tacky and a layer of 80-100gm added. Repeat the heavy cloth for a larger cowl. Airing cupboard again but even though this appears to be set leave it for 48hrs and with the aid of a rubber hammer it will part from the mould, hopefully. Quote Link to comment Share on other sites More sharing options...
Tim Ballinger Posted January 26, 2020 Author Share Posted January 26, 2020 Interesting method Martin, presumably that is epoxy resin you are using for both gel coat and lay up. I had set off with the same intent , remembering how I used to do it, but then saw this polyester resin gel coat which was given a good write up and was brewed for epoxy compatibility. It seemed to work just fine with my first attempt, it was just that I let it get too thin in one place. Hence I tried a pigment next time so that I could see what I was doing. A second coat was warned against with the resin I used as it was said it would not bond properly too itself once cured. i think I might revert to epoxy resin for both next time like yourself. Tim Quote Link to comment Share on other sites More sharing options...
Martin McIntosh Posted January 26, 2020 Share Posted January 26, 2020 Tried polyester once for skinning a wing. Never again, it was just not sticky enough. Quote Link to comment Share on other sites More sharing options...
Paul Whitwell Posted January 29, 2020 Share Posted January 29, 2020 Hi Tim - well that's certainly a couple of decent cowls to play with but the plus is I now have a better understanding of the process and the materials so thank you. One quick question regarding the propellor you used on your model - I will be using a Super Tiger 40 engine as per the original Eric Fearnley plan spec, and propose a 10 x 6 propellor - does that sound about right? Paul Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted January 29, 2020 Share Posted January 29, 2020 Paul, 10 x 6 Quote Link to comment Share on other sites More sharing options...
Tim Ballinger Posted January 29, 2020 Author Share Posted January 29, 2020 Paul, Yep 10x6 is good. Is that ST the same one you bought way back when ? Sorry I got a bit sidetracked this week so have not posted the cowls yet. Will do it by end of the week. Tim Quote Link to comment Share on other sites More sharing options...
Paul Whitwell Posted January 29, 2020 Share Posted January 29, 2020 Hi Dave / Tim - thank you for confirming. Yes the ST is old - circa 1982 so not sure if that makes a difference from a performance perspective? No worries with the cowls - just whenever you get chance is fine Paul Quote Link to comment Share on other sites More sharing options...
Tim Ballinger Posted January 29, 2020 Author Share Posted January 29, 2020 Paul, I am no ace with old 2 strokes and you may be so I apologise if I am teaching you to suck eggs as it were but has it been run recently or just stored for 38 years ? Just asking as it might need some TLC before you fire it up . Tim Quote Link to comment Share on other sites More sharing options...
Paul Whitwell Posted January 29, 2020 Share Posted January 29, 2020 Hi Tim I actually completely stripped it down and cleaned it using a slow cooker and antifreeze (I found a site on the internet which guided me through this) - it came up looking perfect - all but brand new. Following the rebuild and changing a few worn screws (heads), I liberally applied after run oil, and have been spinning it by hand once a week for the last 9 months. Fingers crossed it all runs fine once I get chance to run it - I'm waiting for warmer NE Scotland weather first ........that might well be a contradiction in terms Paul Quote Link to comment Share on other sites More sharing options...
Tim Ballinger Posted January 29, 2020 Author Share Posted January 29, 2020 Paul, Great, looks like you have it covered. Tim Quote Link to comment Share on other sites More sharing options...
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