Fiberglass taping an inside corner

Gosh, what a complication for a so simple thing.
Stay away from polyester first and products like bondo that have nothing to do in wood epoxy boats.
Marshmat has given the right method and Rwatson a very good trick.
Epoxy putties can vary largely in quality and strength, all depends on what you're using as filler.
Wood flour work pretty well with a bit of silica to smooth it. Made a trial on scrap to determine the radius (6 oz. glass cloth can differ in stiffness).

Use the wet method, pretty fast and simple with minimal sanding;

0-prior sand the surfaces to join. If you're messy use electrical cheap vinyl tape each side of the joint. That helps!
1-wet lightly with pure epoxy the surfaces to glue, that insures a perfect gluing.
2-fillet with relatively thick non saggy gooz, like thick peanut butter but smooth,
3-clean with a sharp stick the surplus of fillet mix on the sides,
4- apply your tape dry on the fillet and smooth it,
5-wet the tape with pure epox. Just enough to wet, the weave must stay apparent. A bit of silica may help to control the thixotropy of the resin.
6-if needed brush a bit of resin, just enough for the peel ply. Now the weave of the cloth is filled. Take out the electrical tapes, or you'll cry.
7-let it cure COMPLETELY. A green resin is impossible to sand, and clogs any sand paper.
8-take out the peel ply. You have a smooth surface ready to coat. Do not sand it !.
9-you'll have only to smooth the edges of tape with a sanding block or an orbital sander with 40 or 60 grit paper.
10-coat with slightly thickened resin (silica). 2 coats. You can add pigment.
Now it's smooth as a baby's ***, ready to paint after a light sanding with 80 grit.

 

10 steps is simple? I did it in 3 steps with fiberglass poly and no waiting between steps. Jam the wet rope in the corner, lay the mat over and apply the resin with a paintbrush.

Horses for courses.

 

Yes 10 steps because I've described in detail...and it's enough strong to last many years and not too ugly. Oh yes it's possible to throw a piece of glass and brush some polyester, without sanding before, nor sanding after, and leave the spills. It's the kind of work I'm used to see here in Cancun. That will last a short time, will leak fastly and you'll have in final more work because you'll have to clean and sand the polyester before the ten steps.
After 42 years of boat building I have three certitudes; I'll die one day, meantime I'll pay taxes and polyester resin will never glue correctly any piece of wood or metal.

 

Hey folks, this is a 5 year old thread, revised by a new member yesterday, likely not realizing how old it was.

 

I'm getting old PAR... But the "10 steps" will remain for the sake of the newcomers.

 

i do it differently, in fewer steps;

1. apply thickened epoxy (using milled microfibres and cabosil) using a ziplock bag with corner cut off to apply a neat bead along your inside radius.

2. Using a radius tool (plastic scraper with desired radius cut into it) scrape the bog into a neat fillet and remove any excess so your left with a perfect radius of thickened epoxy.

3. Immediately mix your laminating resin and wetout the fibreglass tape on a bench / table covered in polyethlyene plastic using a paint scraper. You want the glass wetout, but not excess resin so that its dripping.

4. Apply a very thin coat of the laminating epoxy to the area where the tape is going - some like to do this before doing the fillet - either is fine, before is probably better but it means mixing an extra batch of resin first which is why i do it second, and be mindful of not making the fillet mix too thick for poorer adhesion.

5. Take the already wetout glass tape to the job and apply it neatly whilst the fillet is still soft. Work it neatly into the fillet using a paint brush and get all the air out. - This is much better whilst the fillet is still soft and can be massaged slightly with the paint brush to make sure theres no little hard bits sticking up and holding the glass up.

If your gonna do any other work to the area later, such as paint or anything, apply a peel ply over the taped area so you dont have to sand it later.

 

Groper, you skipped the substrate wetout. You can't just apply thickened epoxy to a dry substrate, unless it's previously well coated and toothed. Also the wet method isn't the easiest or neatest method, particularly for the back yard builder or a novice attempting a repair. Step 6 of the 10 (actually 11 if you count 0), is really just part of step 5. Also a few of thesteps could be combined, but over all an acceptable process.

 

PAR, see step 4 of my above post... i mention the wetout of the substrate there, but that way it requires mixing a small batch of resin to wetout the substrate, then the thickened batch, then the laminating batch... I should clarify, *assuming a FRP laminate is the subtrate*, i skip the first little batch and put the thickened epoxy down directly, making sure the substrate is fully cleaned and keyed prior, and the thickened epoxy is not too thick - i make it as runny as possible just thick enough so that it wont sag and loose shape when i make the fillet. I deem it "wet enough" to skip wetting the substrate before the fillet. For timber or other porous substrates, definately wetout with neat epoxy first.

Some may not think its acceptable, but it works fine for me and its done in 1 continuous process... trying to wetout dry glass in-situ is a pain in the ***, messy, and its much too slow when you have LOTS of taping to do. Hence i prefer to wetout the tape on a plastic bench then stick it in place - you also can control the fiber to resin ratio better with zero drips and spilled epoxy on the job.

 

we use the fiberglass rope ( continuos gun roveing method ) then apply a little mil fiber putty with a radius ! works like a charm !

 

Problably it won't be strong enough for a structural fillet chine. Destructive essays will give the answer.

You need a surface cloth or UD to pass the stresses from one panel to the other and enough surface to have a good shear transmission or the joint will delaminate. It's better to have +45/-45 at least so all the fibers work to keep the joint. The sequence of the fibers depends on the expected stresses.
The advantage of the dry cloth or UD on the fresh putty is to smooth the whole thing and to save the episode of sanding a rock hard fiber putty, often in very awkward places.

 

"Some may not think its acceptable, but it works fine for me and its done in 1 continuous process... trying to wetout dry glass in-situ is a pain in the ***, messy, and its much too slow when you have LOTS of taping to do. Hence i prefer to wetout the tape on a plastic bench then stick it in place - you also can control the fiber to resin ratio better with zero drips and spilled epoxy on the job."

It's matter of taste effectively (and necessity sometimes). I find that pre-wetting the glass can be very messy sometimes and the control of air bubbles trapped between the putty and the glass to be annoying.
With big joints needing several layers of glass is almost impossible to use the pre-wetted fiber. Each applied layer has to absorb some of the resin of the previously applied layer, or it will be too "resinous".
I think also that the dry cloth or UD permits to smooth, also it will absorb some of the resin of the putty making it firmer. One of my criteria for buying cloth, UD and resins is the easiness and speed of wetting, so the wetting is obtained by just depositing some resin which will diffuse by capillarity and displace the air not letting bubbles if possible. No need to push-brush. That permits to control exactly the amount of resin.
A resin too thick or a fiber long to wet because of the surface treatment or the weave are a pain.
PS I always protects the surfaces (I pre-finish a lot of things like bulkheads and the protection is mandatory). I takes far longer to clean and sand than to retire a thin plastic sheet as those used by the painters. So no problems of spilling, nor drops.