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Monday 16th, 60F, heavy overcast with heavy, thundery rain possible.
With the negative forecast I am loath to do any fibre-glassing today. I can't stop midway for a sudden downpour. Particularly if I am working on the outside of the dome. I don't want to take the shutters off yet. Because they support the tarpaulin cover over a much wider area. Not to mention having no jagged rib edges to rub the tarpaulin away. As it moves gently in the wind.
I calculate I'll need at least two more litres of polyester resin. I want to make a thicker layer on the external joint between the dome and the slit ribs. This is vital to weather sealing. While helping to spread the heavy shutter loads over the full length and breadth of the slit ribs and zenith board. The ribs and board provide the vital stiffening flanges to the slit cut-out. Though only if they are immovably bonded to the dome itself. The dome is far too thin to provide much stiffness. Except that due its spherical form.
I'm wondering if I can build a dam to contain the fibreglass to a fixed width of dome surface. You can't just apply masking tape. The GRP is far too thick, strong and stiff to allow peeling to an edge. Leaving the edge of the new fibreglass work to its own devices will make an untidy edge of ragged glass matt strands. Though it could be sanded down to make it smooth[er.] There is the serious risk of damaging the original glossy dome surface. A smooth, tapered edge would be stronger. Without stress raising discontinuities of a hard, raised step or lip.
Cutting the CSM with scissors does not guarantee a visibly "smart" edge. It would probably be worse than typically rough fibreglass!
Perhaps I am overthinking the problem. A layer of cosmetically acceptable material laid flat on top of the fibreglass might do. If only to hide the worst of the CSM. Or, the "something" could be bonded to the wet, glass layup. Rubber? A strip of thin, birch plywood? Plastic? A degree of flexibility is essential to match the uneven curvature. What ever "it" was, the strip should provide a crisp, clean edge. Without detracting from the appearance of the completed dome. Glassing over the strip might be worse than having no strip at all!
The shape of the inside top of the dome has been scribed onto plywood to make a pattern. This form will become an internal brace between the lower tops of the slit ribs. I will use 15mm ply to quickly build up the thickness. The zenith board sits just above. Sandwiching the top of the dome in thick plywood. While simultaneously tying the tops of the slit ribs solidly together over their full height. Cross bracing is only possible at the top and bottom of the slit ribs. Otherwise it would block the view of the sky.
Long and hefty eye bolts will pass right through the zenith board. Further helping to clamp the glued and screwed layers of plywood together. The eyes will provide solid support for the pulley system for lifting heavy OTAs. Which is why the 40mm thick, zenith board needs to be glassed onto the top of the dome for maximum strength. I will probably glass-fibre the inside of the lower brace to the dome for similar reasons.
While I was matching the profile of the lower batten I checked the zenith board. It no longer matched the top of the dome exactly. So I used the band saw to trim it to a newly scribed line. Followed by the angle grinder with a coarse disk to smooth the cuts. While I had the zenith board removed I brought it flush with the edge of the dome cut-out. Then re-fixed it with the perforated roofing plates to the slit ribs. The image above shows the results.
Once the full thickness of the lower brace is achieved I shall fix it flush with the dome cut-out. The brace can then be fixed to the zenith board with more perforated plates. The eye-bolts can pass though these plates to avoid crushing the plywood when the nuts are tightened. Though I shall use large, load spreading, roofing washers anyway.
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