1985 P 18-2.
Is it common for 35+yr.old Prindle decks to "oil-can?
Tried the "Hobie" method, drilling a regular pattern of many 1/8"-5/32" deep holes, injecting polyester resin.
Result: did not stiffen deck. Following this failed attempt:
Here is my next attempt at fiberglass repair:
Did cut out fore decks with an inch remaining inside of the seams which join to the hulls, from 3" before front spar, to about 3" abaft the forestay chainplates.
Appears Prindle decks' construction is not the same as "Hobie". Looks like: gelcoat, layer of woven cloth, layer of foam, thin layer of fairly porous "cheesecloth"/resin. So, my injections just dripped through, and, did not spread laterally. Prindle decks on at least my example, do not have much of an inner layer.
Ground smooth the underside lip edges of hulls, cut 3" wide strips of 1/2" Canacore ( because I didn't have any 1/4") to form an internal ledger board completely around opening in hulls; in sections( one for each side and end).
Cut chopstrand strips 1-1/2" wide, to hold resin, coating the outer half of ledger boards wi/ resin, rolled resin on underside of hull edges, put wood lathe strips along and under each ledger strip, used numerous spring clamps to hold ledger boards in place until resin cure, removed clamps & temporary lathe stiffeners.
Cut 1/2" Canacore to apply underside of cut out decking sections. Decks are convex! Used razor knife to make many parallel 1/4"-3/8" deep slices lengthwise on upper side of core, to allow it to conform to deck curve. turned deck segments upside down, layer of chopstrand and copious resin to adhere core material to decks' underside. used 2"x4" & a brick to hold core into curved deck underside 'til resin cure.
Then laid on layer of chopstrand and resin to underside of core. Once cured, did minor trimming such that reinforced deck segments would lay flush in hull openings. Applied 1-1/2" strips chopstrand onto exposed ledger board edges, and bedded deck sections into chopstrand&resin.
Having ground off gelcoat on the repaired area, my next of many questions:
Just reapply gelcoat to the repair area? Or, apply layer of woven cloth/ resin to further ensure the "integrity" of the hull?
Moving right along.
The decks between the forward and after spars are somewhat soft as well. This area has the dagger board trunk. The prospect of duplicating the complicated method I used on the foredecks seems both labor intensive, and perhaps, unnecessary, or even ill-advised.
Second question:
Having ground off the gelcoat in these areas, am wondering if merely laying on chopstrand to hold sufficient polyester resin to adhere a layer of woven cloth soaked with resin, and re-gelcoat, will adequately stiffen those deck areas?
Welcoming any insight/constructive criticism here.

You are going to hit the wall of diminishing returns here on a 35 year old Prindle with soft deck issues. Perhaps seek a replacement pair of hulls. I think laying new glass over a soft deck is going to be good money after bad.
Did you reattach the top layer of the deck panels that you cut out up forward?

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'82 Super Cat 15
Hull #315
Virginia
Previously owned: '70 H14, '79 H16, '68 Sailmaster 26, '85 H14T
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I generally agree with the advice to look at replacement hulls, which effectively means replacement boats in this size range. A Hobie Tiger comes to mind. Well built boat, good hull shape, replacement parts are available, and the base boat will have parts that are maybe 15 years old, not 36+ like your prindle. This assumes you value your time spent on boat repairs...the work you are talking about starts at $80/hr in the area I live...I'd want to make sure my beams, rig, rudder system and boards were in good shape before proceeding.

Second, if doing this, ditch the chop strand matt and polyester resin. They are poor choices for materials and the original hull was at least built with woven materials. I would pair 6-9oz S-Glass woven on the biax with resin research system 2050 resin for a deck repair. You'll also want plenty of peel ply. There is a myth that gelcoat doesn't stick to epoxy, it does after a nice 220 grit scuff. The other option is a single or two part poly paint after finishing (I like Awlgrip, but it is $), rolled and brushed, then covered with EVA nonskid for most of the deck. Rolling and brushing doesn't produce a mirror-like finish but you don't NEED one, since EVA will hide most imperfections and provides for a nicer deck surface anyway.

If the top deck is getting soft, one should understand why. Is there a seam that is failing? Is the core going bad? Was the deck laminate simply too soft to begin with and 35 seasons of crew slamming into it showing up as a soft deck (BTW, this is more common than you may think, even on never boats).

Likely the "right fix" is to route through the bad areas, just the top laminate and the core, living the thin inner skin. Sand and potentially apply a tighter weave of class to the inner skin. Replace the core. Re-glass the outside to probably +25% or +50% the original laminate thickness, depending on materials used. This all requires care, with the size repair another pair of hands comes in handy, and the use of a vacuum bag would be helpful.

Thanks for timely & relevant responses. Acknowledge being in diminishing return domain. got $1,100.00 P18-2 three years ago. have put more than that into all standing and running rigging.
Sails, battens, trampoline, spars & trailer all seem good.
Have enjoyed some lake sailing, and Columbia River. Really don't enjoy trailering the 8'6".
In preparation for Columbia River sailing- Portland, OR. fabricated a battery box hanging from front cross spar, found a used electric trolling motor. Installed folding Danforth anchor. This to both avoid commercial traffic, and to make away from piublic pier and head up during adverse winds, thus to raise main in mid channel. Have harken roller jib.
The soft decks were uniformly worse on the foredecks My reinforcement strategy has worked . the after deck regions, plan to build up with 1/4"canacore, cover with woven layer. & gelcoat. Liked the EVA suggestion. Again, thanks.
fastedde
Prindle 18-2

Skip the canacore, at least from what I can see it is a plastic honeycomb. Bonding to that product is a serious issue (see Nidacore failures in the past). I don't advise the use of any honeycomb materials for home repairs, even in an ideal environment they can be a challenge to bond to. They are really meant for the use of pre-preg materials.

Instead, go with Divinycell, in 5 or even 8lb density: https://www.fibreglast.com/product/vinyl-foam-5-lb-density

The 1/8" would probably work fine on a deck depending on exactly how you attempt the repair, in this application I think more glass is better than more foam for higher compression resistance (and you aren't as sensitive to weight; if you were, I would be suggesting a different course). I also tend to use a roller prick (https://www.amazon.com/MAGIKON-Leather-Sewing-Spacing-Roulette/dp/B00LHVOLCY/ref=asc_df_B00LHVOLCY/?tag=hyprod-20&linkCode=df0&hvadid=198099762925&hvpos=&hvnetw=g&hvrand=12390699718199985514&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9008187&hvtargid=pla-319275923960&psc=1) to put some holes in the foam, which allows epoxy and vacuum to flow from one side of the foam to the other and forms some epoxy rivets in the final layup, this helps keep the skins bonded.