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A sticky glassing problem!

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Allan Bennett30/03/2019 20:58:04
1563 forum posts
43 photos

I finished glassing a built-up balsa wing a week or so ago, and the epoxy resisn is still sticky in places. It's dry and hard, as it should be, everywhere except where two layers of the 25g cloth have been overlapped, such as on the leading edge, wing tips, and a join on the centreline. Underside of the wing was done first with one batch of resin, and a couple of days later the top side was done with another single batch. A few weeks prior I made a mould and then a cowl using the same resin in the same proportions, though with 50g + 200g cloth, and it hardened right through. All items were kept indoors to keep the temperature above 20 degrees.

After noticing the problem I then noticed that my resin is 3 months beyond its use-by date, so I've bought a new lot for future glassing. But what can I do about this wing -- leave it for another couple of weeks and hope it will harden, or use a solvent to wipe off the stuff that's still sticky, and then re-coat with the new resin? Or is there any magic 'kicker' like we use for cyano, that will initiate the final hardening?

Peter Christy30/03/2019 22:34:02
1615 forum posts

If Sunday is as sunny and warm as Saturday has been, try leaving it out exposed to the sunshine. That has worked for me in the past!

Alternatively, a hot air gun (as used for heat shrink covering) or an Anglepoise lamp with a hot bulb in will accelerate the curing time.

Keep an eye on it if you put it under a hot lamp, though!



Percy Verance31/03/2019 08:53:27
8108 forum posts
155 photos

Oh dear. This is exactly why I gave up on epoxy resin on glass cloth. Poly C all the way for me. No mess, no smell, brushes clean under the tap..... yes   And with a glass cloth wing bandage it's aliphatic resin for me. Epoxy is messy, glass resin stinks. I don't like either of them. Get some Poly C, you'll never look back.



Edited By Percy Verance on 31/03/2019 09:05:42

RC Plane Flyer31/03/2019 09:40:18
661 forum posts
19 photos

Sunshine should dry it I always cover the epoxy with masking tape but jointed so the solarfilm or coverings stick to it better

Cuban831/03/2019 10:06:51
2809 forum posts
13 photos
Posted by Percy Verance on 31/03/2019 08:53:27:

Oh dear. This is exactly why I gave up on epoxy resin on glass cloth. Poly C all the way for me. No mess, no smell, brushes clean under the tap..... yes And with a glass cloth wing bandage it's aliphatic resin for me. Epoxy is messy, glass resin stinks. I don't like either of them. Get some Poly C, you'll never look back.



Edited By Percy Verance on 31/03/2019 09:05:42

Epoxy doesn't suit everyone and I'm pleased that Percy has found an alternative that suits him. However.......epoxy is not necessarily the devil's venom that he makes it out to be and it does give an excellent finish that accepts all manner of paint. When applied and correctly cured it remains stable for years. I'd never touch polyester resin unless I was making a cowl or something similar and could work outside. All my epoxy glassing is carried out indoors, because it has very low odour and is not (to me) unpleasant. Don't get it on your hands though, it will irritate your skin. As for mess, well, it's just a case of good working practice. I'm not a particularly tidy worker (as Mrs C8 keeps reminding me) but I do get my act together when glassing.

I don't know why the OPs resin hasn't gone off fully. Several possibilities - stale resin, improperly stored resin, contamination, poor mixing, inaccurate measuring and a few others. I've only had one instance of that problem years ago with SP113 and I suspect the stuff had been sitting around on the model shop's shelf for ages. IIRC it was on a wing bandage for a foam wing and I think I got away with the problem by wiping the job down fully with a meths soaked cloth several times and then recoating with fresh resin, keep the job nice and warm for a few days and with luck, you'll be ok. Not ideal though, and on an area that requires a good surface finish, you might not get away with it.

I always use resins that require a 1.1 mix to keep things simple and my product of choice is from Bob Smith Industries, that you can get from various suppliers and even Hobbyking. I will give Poly C a try, I'm not averse to new techniques, and the water based nature of it sounds attractive.


Edited By Cuban8 on 31/03/2019 10:17:15

Levanter31/03/2019 10:33:44
881 forum posts
436 photos

Hello Alan

Echoing Cuban8 and adding my own experience with composites, the two most likely causes of an improper cure (discounting temperature because you covered that) is inadequate mixing and inaccurate measuring. The proportions of resin and hardener are critical sometimes down to just a few percent. Actually resin and hardener can be a bit misleading. Adding more hardener does not make it go off quicker or harder for that matter and the chemistry is totally different from polyester resins using a catalyst and an accelerator. In epoxy they are components of a two part mix and many industrial application simply label them component A and B.

Mixing small quantities and you need very sensitive scales otherwise it is best to use volume dosing.

Cuban8 makes a good point about having a 1:1 mix as that makes mixing any quantity dead easy and helps avoid mistakes. I was making an epoxy component for an RC model yacht a short while ago and used a 1;1 mix instead of a 3:2 on the instructions. It did eventually cure after applying as much heat as I dared over a long time (many days) but never went hard. It would have taken a paint finish but as a structural component it was useless.
Try the heating as recommended above and good luck.


Don Fry31/03/2019 11:08:09
4117 forum posts
48 photos

As you say, be careful about the dose rate with epoxy. I have found it's tolerant of ageing ( good starage is a must).

Poly C I have found is easy to use, but third rate for strength, adhesion, and fuel proofing. After all it is a fish pond sealer is it not.

Allan Bennett31/03/2019 11:33:28
1563 forum posts
43 photos

My mix ratio is 1:5, and I use graduated beakers to make sure I get it right. I must have got measurement and mixing right for, as I said, it's cured okay over most of the wing surfaces -- it's just overlaps that are still sticky -- and it was all from a single made-up batch of about 25cc. As I said though, I noticed later that my resin was a couple of months beyond its use-by date.

I'll try sunshine, when it comes back, and maybe my heat gun. So if that fails, you reckon meths will remove the sticky bits so I can re-coat them?

Incidentally, I like epoxy resin because, as has been mentioned, it's relatively odour-free and performs well, sticking to almost any surface. I'll have a look at poly C when I finish the new can of resin I've got, but can it be used for making up cowls etc., or do you just use it for laminating?

jrman31/03/2019 11:59:13
349 forum posts
3 photos

Allan, I have and still do use resin which is well past its "use by date" ( years not just months) with no apparent problems. It's stored in my workshop with no special treatment. The only problem I've ever had (fisheyes) was with some SP113 a number of years ago when Ripmax apparently changed their supplier and manufacturer and ( according to the original manufacturer probably altered the recipe). I would recommend swabbing with Isopropylalcohol in lieu of meths' which can leave a residue (blooming) on the surface. Gentle heating is also a good idea as the problem seems to be on the overlaps where the laminate is marginally thicker.

Edited By jrman on 31/03/2019 12:02:22

David P Williams31/03/2019 12:12:34
862 forum posts
282 photos

Don - PolyC isn't the pond sealer stuff as far as I know. It seems the same as Deluxe Materials EZE Coat, like a clear polyurethane varnish with added stickiness (PVA?). I've used it for ages for glass coating sheeted airframes and it works really well and as Percy says is easier, odourless and cleans up with water. It seems to be fuel resistant rather than fuel proof as claimed but my glassed surfaces are always painted and fuel proofed anyway.

Martin Harris31/03/2019 12:30:10
8945 forum posts
221 photos
Posted by Don Fry on 31/03/2019 11:08:09:

Poly C I have found is easy to use, but third rate for strength, adhesion, and fuel proofing. After all it is a fish pond sealer is it not.

I thought it was Ronseal Diamond Hard floor varnish. When I've heard about pond sealer it's normally been referred to as G4 Pond Sealer which is a moisture cured polyurethane - both are polyurethanes but while Poly-C brushes are washed out in water, G4 requires acetone or cellulose thinners according to the maker so they appear to be rather different.

I do recall reports that someone damaged their lungs very badly by spraying G4 onto a model so take care!

Don Fry31/03/2019 12:31:30
4117 forum posts
48 photos

I think it is, or has its roots in the pond sealer stuff. But end of day, it isn't fuel proof, and is a poor adhesive in comparison to epoxy. If it was that good, given that it is cheap, water based, non poisonous, boat builders, as in full size, would use it.

To mix epoxy I use electronic drug dealers scales, 0 to 200 grams, in .01 gram increments. You can get them for less than £5.

Edited By Don Fry on 31/03/2019 12:35:49

Bob Cotsford31/03/2019 13:26:20
8059 forum posts
444 photos

My problem with Poly-C is that it needs so many coats to fill the weave of even lightweight glasscloths. PolyC/glass gives a more resilient finish than epoxy in that it flexes more, but it does appear to peel off more easily as I found out when examining the remains of my Poly-C/glass covered Joker. I wouldn't call it's adhesion poor, just not as good as epoxy, but if it's bonding to balsa either will be stronger than the wood itself and will pull away the outer wood fibres rather than peel away cleanly.

Use it for glass mouldings - I've not tried it but I'd guess you would be painting on dozens of coats to fill the weave on heavier cloths and I suspect shrinkage would be a major problem. Epoxy would be both quicker and stronger in my opinion.

Sorry Don but I too believe it to be re-packaged Ronseal industrial wooden flooring sealant/varnish. I also believe it gives off something in addition to water when drying, as a COPD sufferer I can detect airborne chemicals down to parts per million levels!wink

Martin Harris31/03/2019 18:43:58
8945 forum posts
221 photos

One thing to beware of if aiming for lightness is that some very lightweight glass cloth simply has fewer threads per inch, meaning that you actually use more epoxy or Poly-C to fill the weave than with the heavier cloth using the same thread size.

John Duncker31/03/2019 20:07:33
79 forum posts
7 photos

My boat came with 1/2 a gallon of West System resin and some slow and super slow hardener. We guys who live in the tropics do have some problems you know.

It would have been 10 years old when I passed it on and still useable and set hard although the hardener was black.

Some epoxies are very fussy about the mix ratio get it wrong and it never gets past the sticky chewing gum stage.

A set of electronic scales are cheap as chips and enable you to get the mix crack on.

TOP TIP 1 Make up a little chart which gives the matching weights eg assuming 5 to 1 then 7g hardener goes with 35 g resin

TOP TIP 2 lay a sheet of cling film over the scales.Discard if it gets sticky and replace.

Allan Bennett31/03/2019 20:24:27
1563 forum posts
43 photos

My epoxy is West System 105, with slow hardener. As I said, I use graduated measuring cups to mix it, but maybe some 'drug dealer' scales would be more accurate, given the inaccuracy that can be caused by viewing the meniscus from the wrong angle.

Interesting to hear your experience with old epoxy John: Since mine set hard a couple of weeks ago for my cowl, and has set hard over most of my wing, I think I can rule out age -- and mixing ratio for that matter. Something else must be going on.

The weather forecast is for sun for a few days, so I'm going to leave the wing outside and, if it's not cured by next weekend, I'll get some isopropyl alcohol or meths (whichever is easier to get hold of locally) to wipe the sticky bits off. I've got plenty of other jobs to do in the meantime!

Gary Murphy 101/04/2019 09:36:55
382 forum posts
14 photos

I have seen vids on youtube and "how its made" on TV where people are using glassfiber . all seem professional places and some have full protection on others not even a mask. is this the resin,epoxy,ployeaster types?

Cuban801/04/2019 12:42:19
2809 forum posts
13 photos

Gary, I've seen those programmes as well and the same thought has occurred to me. I don't know the answer. What I can say through experience, is that Polyester resin is nasty and unpleasant, is very bad for the skin and will give you a headache or even dizziness if you breath in the fumes for too long. I like polyester car body filler, but I'd only use it in the garage and with the door wide open. It stinks and the smell lingers on your clothing for ages.

I've mentioned before that a friend of mine who used to make polyester glass fuselages for one of the big name suppliers in the 1980s, did serious damage to the lining of his nasal passages because of working in an enclosed, heated room for extended periods whilst laying up glider fuselages. I don't know of any problems associated with the sensible use of epoxy resins, other than by prolonged skin contact. AFAIA, I've not suffered any problems when working indoors with epoxy from its odour, and I have to say that I've not always been particularly careful about ventilation during winter.

Piers Bowlan01/04/2019 14:45:59
1905 forum posts
50 photos

Repeated skin contact with epoxy will in time cause sensitisation and allergy, resulting in dermatitis. Some amine hardeners are carcinogenic and can also cause asthma. As you say, just take sensible precautions like avoiding skin contact and working in well ventilated areas (or wear a mask). And you thought that only a spinning model aeroplane propeller could be dangerous!

Edited By Piers Bowlan on 01/04/2019 14:48:58

Martin McIntosh01/04/2019 18:45:51
2970 forum posts
1090 photos

Having been bitten a few times by some brands of epoxy I now only use Multipurpose resin from ABL Resin and Glass in Sandbach. Easily mixed 2:1 by volume but you really do have to mix it well until the result is clear. I use this for moulding and skinning. (Avoid their General Purpose stuff though.) It always sets hard enough to sand easily after 48 hrs.

Very surprised that the thicker overlapped areas will not set since these should be the hardest; as has been said above this is probably due to insufficient mixing. Acetone dissolves epoxy so a wipe over with this may remove the stickiness, otherwise just re coat the affected parts.

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