minor deck core repair help

I was thinking at least Polly board. On smaller Jobs, I have had great results with West epoxy and 406 Colloidal Silica Filler;
Make a peanut butter with some slow hardener at first, just start pushing it in against the coring. After you get some way in there, tape off the bottom of the hole, and start applying more peanut butter epoxy from the top with slow hardener. That is a thick chunk of epoxy and you do not want it to flash fast and get hot. If the epoxy runs, it's not thick enough. A job this size is going to take several small batches and over an hour. Don't rush or answer the cell phone. The next morning you may be able to remove the tape and shape the hole from the bottom, The epoxy will not be completely hard so avoid getting back on the top for another day. The second day, sand, file and drill as needed. Bed & Bolt. Never worry about it again.

You will want to use the better gloves from West or heavy mechanics gloves. the thin/cheap ones will tear in a few seconds.

Forget foam or cheap bondo like resin, stick with epoxy which adheres better to existing stuff.

If you are sure that you have removed all the wet rotted wood, and if the void isn't too deep, you can fill it with thickened epoxy. I like west system thickened with their high density filler. Easiest way would be to temporarily seal the bottom of the hole (duct tape on the ceiling) and force the thickened epoxy in the gap. A few voids will not be a problem...

If the volume to fill is too big then you have to replace the core. Mahogany is an option as it is a hard wood and resistant to rot but it is pretty heavy therefore not ideal for large surface.

The best core to use would be divinycel but it can be hard to find just a small piece. You could cut in pie slices and push them in one at a time after coating both sides with thickened epoxy. If you can't find any modern form core, you could use some marine plywood. Make sure the edge is recessed by about 1/4" and fill that up with epoxy so that water water can't get back in

From the pictures, it looked like foam core. Just back a tad in one direction and a ways in another direction. Scrape out some of that foam around the thru bolts a ways also. The 406 filler holds up to compression very well. Just take your time and use the slow (206?) hardener.

Even if that is wood, clean it out and do the same, Fill er in..

Definitely balsa core... Let it go to long and you will see core juice leaking on the edges....

Being in Oregon.... The slow hardener may not work because it s cold up there... You may need to stick to the fast 205 even with the volume.

Basically if the diameter of the hole is not larger than the void, I would use thickened epoxy. If the void is bigger than the hole, then you ll have to use core. The good thing about using down is that it is a little flexible so you should be able to gets it in whereas wood will not bend enough.

Core repair

Another technique that helps is; after removing the decayed area, saturate the remaining core area with acetone, which will displace any moisture and quickly flash off. Then make a small batch of epoxy and slightly thin with acetone and paint this into the area as a bond base and let cure. Then apply the collodial "peanut butter" and you will have solid monolithic repair. I have used this method on dozens of mahagony soft areas with great success.

I'm in Jax now working with the stuff, down to low 40 up to mid 60s. The slow stuff is working great on some thick holes. It does take 2 days before it's hard.
In the past, the regular cure after 1/2 inch in thickness got hot. That just makes it brittle later. Like I said, Take your time. It will set and will not crack in a few years (weeks).
The West web site is full of tips and used to have great videos. If that is a high stress point, research and do it right now. A common goal from all of us.

Morning,
Taping the bottom of the hole is a good idea and I would add, after applying tape, put a piece of wood larger than the hole over the tape and put a jack (car jack for example) under the piece of wood and apply slight pressure. This will eliminate the possibility of you pushing the tape off while filling from above.
Peter

In Engineering there are different ways to mount or add strength to whatever. Like tension, suspension & Compression. You have a failed compression mount. The inner hole is 3", get a 3" hole saw. Then get a piece of hard wood at least as big as the outer water marks & the correct thickness. cut it round,6 to 8 inches or whatever you can dig out. Drill your 3" hole in the center, then cut the wood into 4 quadrants so they will fit into the 3" hole. You may have to half some quadrants to smaller pieces to get them between. Be sure each section is lined up under the through holes. Epoxy them in Drill & Install bolts and nuts through the hardware and you will have a strong compression mounting, as long as the upper and lower glass is still intact.

I'm going to agree with wzrdl. That area is a lot more structural support to the davit than everyone thinks. I wouldn't just pack it with mud. Persornally if it was my boat I'd cut a 1' square off of the top layer of fiberglass, dig out the coring and rebond/glass new coring to the lower fiberglass and reglass the top 1' I cut out with new glass.......

The best for engineering strength and reliability of both doing the job and long term... is the solution by Pascal.

Do not try to piece in core. As the skins are good and the surround core good and the failure was the core... the core was not up to the job.

The compressive strength of wood at best, I will do english units hear because you would have to convert metric anyway.

Wood varies between 700 to 2000 lbs per sq. inch compressive strength

The West Epoxy with a heavy load of cotton or glass flock fill is at least 10,000 lbs per sq. inch compressive.

Not only will this fix the structural problem but will fix the adhesion and long term reliability significantly better than and pieced wood or other core repair.
And the reliability of accomplishing a solid and water proof repair bonded well to the skins and the good remaining core is higher.

Flashing off with the acetone is really good as suggested hear after all the damage or rot is removed. Make sure the underside of the repair is well sealed and supported before pouring in the epoxy mix from the top.

Warning be sure to follow the manufacturers guidelines on the situation of a large fill pour.... as if you use too "hot" of catalyst you could cause the epoxy to overheat due to the cure chemical reaction... in fact you could have it catch fire. These means any epoxy that has a quick cure time is too hot to consider... use one will a very long slow cure time and it will because of the mass of material cure much quicker than stated.... call West System and explain or refer here.

DO NOT USE EITHER POLYESTER OR any resin other than epoxy or a casting urethane that is rated for a fill type "potting".

I'm looking forward to the OP's final plan. Lots of ideas offered but I think the epoxy idea has been explained pretty well from a few different contributors. Gad I love the stuff.
,rc

A couple final comments. Epoxies are available in "potting compounds" already mixed up. But they are usually available from speciality compounders... check your area. And, they are formulated for that use. All you have to do is mix in the catalyst and 'splodge' in.... like with consistency about like butter to a taffy... or even thicker.

When you use West System or other bonding and layup resin systems and mix in floc you will end up with something be quite runny more like syrup... unless you use what you might consider huge quantities of floc which is why usually the compounders are better equipped to make a potting compound... the mixing problem. A liter off resin might be used with a surprising quantity of floc... much more than you would think... and it is hard to mix in.

Certainly you can mix up your own like using the West epoxy... but the issue is choosing the correct "b" component or catalyst. West and others are formulated for hobbyist or commercial operations that are not hugely sophisticated. This is why the mixing ratios are near 50-50 they are planned to be easily proportionally measured or metered.

AND, they are formulated for layup and bonding for thin glue or matrix thickness... not for thick sections. This is why the you can get exothermic reactions with high heats or even fires created when used in large concentrated quantities. Your potting situation is a large concentrated quantity. So you have to make sure with the manufacturer for his recommendations.... which might include making several thinner pours.

Most epoxide systems used by larger commercial operations or aircraft manufacturers are not so simple. But the engineering staff is present to determine the best chemical solutions for the particular situation.