This isn't a shot at you, this is a common topic when refinishing with gel coat. So please don't take offense at it. It is something I've posted before.
Uneven cure can be caused by a few things, but the chemistry isn't smart enough, nor does it have the ability to cure in one area and not in another without some outside assistance. This is totally unrelated to adding Duratec, it's something that gel coat and resin tech service people get calls about fairly often.
Frequent causes of uneven cure.
Not mixing the gel coat or resin prior to using it.
-Always mix before use or beyond an hour since last use
Using old gel coat and/or catalyst.
-I purchase small quantities frequently. Typically 1 gallon jugs which are easy to shake.
Not mixing the catalyst in thoroughly. Always mix it in a separate container before pouring it into the gun. And mix for at least one minute with a good stir stick. Or better yet, a drill motor with a paint mixer attachment. This is vey important and a common problem.
-For the task I referred to I mixed in the gel coat and Duratec in a clean metal paint can. Mix using a whisk for two minutes. I then took the mixed product and poured into paper cups to add hardener. Mix for another minute. I don’t time but count to sixty+. Maybe this is where I went wrong?
Not using the correct amount of catalyst. This compounds the poor mixing issue.
-Two percent according to Duratec. Hardener providesd with GC is Norox I don’t recall the product number but my supplier says it is compatible for resin and gel. Have never had any issues with bad or extended cure times.
Not having air moving across the surface. Styrene fumes can collect in low spots and inhibit the cure.
- Panels were suspended vertically
Uneven thickness on the surface, thicker areas will cure quicker, some thin areas may not cure well at all.
- Pretty sure my spray cover was equal. Sanding the product off seemed to confirm this. No runs, drips or errors.
Temperature differences across the surface.
- work was done in a garage, panels were hung night before but heat was run minimum of an hour after 70 to 75 degrees was achieved.
Sun, or other heat sources affecting the surface. In direct sunlight wax may not come to the surface prior to the gel coat starting to cure, so the surface will remain tacky. This can be uneven across the surface depending on the heat source.
- No sunlight exposure
Some types or colors of gel coat cure poorly with any additive, every gel coat company has hundreds (actually thousands) of different formulas, each one can cure differently.
Some colors don't cure well. There can be a huge difference in how each pigment impacts the cure, and the bolder the color, the greater affect it can have.
- Again GC was Ashland standard white non wax provided by manufacturer. Perhaps their pigment is not compatible with Duratec?
Since 99.99999% of all gel coat is used in-mold, that's where it is formulated to work. The fact that it can be post applied like a paint (most of the time) is something that isn’t targeted in the R&D work.
-Yup that would be the norm but, the GC cures just fine w/o the Duratec
It isn't uncommon for some gel coats to cure poorly with any of the surface cure additives, so frequently two additives can be combined to aid the surface cure. This can be done by the person doing the work, or by the manufacturer. Duratec offers several products that have both wax and the surface cure resin in combination to achieve a quicker and more thorough surface cure. With Duratec alone you get a very high gloss finish that will level and flow more like paint, some people don't touch it after that, no sanding or polishing, that's the finish they want. Others will sand and polish this finish to make it even smoother. If you plan on sanding, then using wax in combination with the Duratec additive gives a you very smooth surface, but no gloss. And since you planned on sanding and buffing anyhow, the initial non-gloss finish is irrelevant.
-True my reasoning for using Duratec was to perhaps achieve a decent look without further or at least minimal after treatment.
Adding Duratec can reduce the amount of sanding required immensely, sometimes you can start with 800 grit and then buff it. With just wax you typically need to start with a much coarser grit to get rid of the orange peel, so the time and cost go up significantly if you want a smooth and glossy finish.
Most gel coat finishes don't improve much past 1000 grit sandpaper, and with some you don't need to go past 800 to remove all the a sanding scratches. This depends a great deal on the exact gel coat and the exact sandpaper. Some gel coats are harder than others, so it's not as easy to buff out sanding marks, softer gel coats are easier to buff to a high gloss.
- The “fix” for my panels was roll with 3/8” nap then tip (brush) run lightly over to knock down the ripples. Result was a fairly smooth finish which enabled me to start with 120 grit to get down to a smooth flat surface with minor blemish. Then to 220 or 400 grit depending where the 120 left off.
Sand paper comes in many styles and qualities, cheaper sand paper may not have the same uniformity of grit size, or may have uneven layout on the backing, so it will leave uneven depth scratches.
-Big box home supply paper which is at the very least giving me the results I want
The hardness of the grit comes into play too. Some grit wears down quickly and leaves finer scratches as you sand, others are harder and continue to cut at about the same rate. Cheap sand paper can have a mix of grit that may have a different hardness, so the scratch depth can vary, and change as the grit wears down. So cheaper sand papers may require continued sanding with finer grits to remover all the scratches from the grit irregularities.
- Some visible scratches after 1000 so went the extra mile with the 2000. And their deck panels so some scratches from typical use are to be expected,
Buffing compounds and techniques make a huge difference too. So you need to find out if you like sanding less and buffing more, or sanding more and buffing less, both methods can work.
There are fast cutting buffing compounds designed to remove 600 grit scratches on most gel coats. These compounds don't leave a high gloss, it will be glossy, but it can be made better with a slightly less aggressive compound being usdd after it. Then can switch to a foam pad and bring the gloss to a much higher level.
If you sand more with a finer grit you probably won't need to buff with the fast cut compound, you can start something less aggressive, and depending on the desired result, you may only need to use this one buffing compound.
Color can make a big difference in the amount of work required, white won't show many defects in the surface, but black will show every defect, they will stand out immediately. And the higher the gloss, the more defects you will see.
