Reinforcing GF cowl

[Simon Lumsdon 1]

Hello, need some advice pls

 

 

Have a large quite flimsy GF cowl for a large old Mustang ARTF kit which i had to cut along the underside to fit, feels like it needs another layer of GF to stiffen it up somewhat

 

i have various cloth weights and z poxy finishing resin, brushes and some experience

 

question is how to glass out the inside of the cowl without spoiling the outside and getting the cloth evenly spread? 
 

My first thought is to brush expoy onto inside of cowl and then lay pieces of cloth into cowl but experience warns me this will end up a sticky mess

 

all advice welcomed

 

thanks

simon

Edited September 23 by Simon Lumsdon 1

[Allan Bennett]

The cowls I've repaired have needed a full scrub-down and repaint, so I haven't really thought about how to protect the outside 😃  I would suggest applying masking tape over the outside if you need to keep it clean while glassing the inside.  Note that it's best to use epoxy resin (such as Z-poxy) rather than polyester when covering existing epoxy, for it will stick to either type.  Personally I prefer to lay the fibre mat first, and persuade it round curves with my finger, before stippling the resin into it.  After the resin has gone off I use P38 body filler for any holes.

[J D 8 - Moderator]

  Like you I have found  layers of glass fiber in thin cowlings does not work well over time. If you rub down through the shiny layer enough for a good grip one weakens the cowl anyway. Just a bit of a key and after a while with the flex of the cowl the glass mat edges start to peel

On both ABS and light glass fiber cowlings I have used "rhino" tape. Rhino is just one makers trade name for that very sticky tape with fibers criss crossed in it. A run of CA around the edge helps seal. On smaller stuff have just used masking tape and again sealed with CA this time all over.

       

[John Rickett 102]

The first task will be to make sure the cowl is scrupulously clean, no silicon remaining from the original mould (unlikely on the inside) and no oil or grease (much more likely given it’s a used cowl).

Wash the cowl in soapy water and then when dry, go over the thing and particularly the area you wish to cover, with a chemical degreaser (panel wipe).

100 gm cloth or thereabout will be easy to apply and adapt to the shape. Two or three layers may be needed depending on how flimsy it is. The good thing about fibre-glassing is you can tailor the amount needed to specific, ie very flimsy, areas.

Brush a coat of epoxy onto the inside of the cowl and then after laying the cloth, wet-out the cloth by brushing more epoxy if needed. Add the second or third layer as you see fit, without seeing the job its impossible to gauge how many layers will be required though I doubt if you’ll need any more than three. Yesterday I moulded a wheel spat using three layers of 100 gm cloth and its more than adequate, weight up the front though is usually of little consequence.    

Keep the ambient temperature to around 200C, you’ll get extended curing times with lower temperatures.

That’s all there is to it, epoxy resin is safe and equipment used can be cleaned with standard thinners.  

[J D 8 - Moderator]

        Hope you mean 20 degrees C John. 😁 200C 🥵

[John Rickett 102]

Oops - well spotted! Yes, 20 degrees.

[Ron Gray]

Carbon tow also helps stiffen flimsy areas.

[FlyinFlynn]

If you watch any of the boating videos on youtube you will quickly discover peoples best practices when glassing over old GRP.

 

The only thing I would add to John's excellent post is use liberal amounts of clean acetone and a lint free cloth as your chemical degreaser.

 

 I would dry fit a piece of cloth to your cowl, making cuts to assist the cloth to conform to the required shape, then remove the cloth and brush on a layer of resin to the cowl, then 'wet out' your pre-shaped cloth on a plastic sheet and then apply the wetted out cloth to your cowl. Remove any air bubbles under the cloth and leave to dry. If you can catch the cure when it is still green you can trim the excess cloth with a scalpel blade, otherwise cut it with a grinder and sandpaper when fully cured (best to wear a simple mask) . You can also place the dry cloth on your cowl after brushing the resin on if you prefer but you are more likely to get in a mess if you are dealing with sharp corners.

 

Use epoxy and not polyester or vinylester resin for maximum compatibility.

 

[Edgeflyer]

Keep in mind that it's very easy to remove epoxy stains before it's set using some methylated spirit