Thanks for the advice. Learning as I go. Hadn't thought about using clear nail polish to fix knots.May I suggest that masts are fixed and maybe wedged into appropriate position.Delay tying until most ,if not all rigging at any one stage, is ready. Maintain tension with pegs or heavier spring clamps, and fix knots with something reversible such as nail varnish/acetone or nitrocellulose/thinners or even dilute PVA glue/alcohol. Natural strings have a degree of elasticity and retightening is often necessary-overtightening must be avoided and certainly not used to alter mast or yard misalignment. Cut the surplus ropes as late as possible. Your idea about a marker to colour is good, I am running low on black thread.
Remember to remove watch and bracelets and roll up sleeves!!!