CHUG'S THE MAN! His suggestions here are spot on!
I've never mixed both silica AND micro baboons. ...er BALLOONS.
But I have used each individually. I have one frame on my Zip that was ever so slightly made inside of the actual dimensions in the patterns - a simple mistake that didn't show up until later. This meant that I had a dimple on each topside that needed filling and fairing. It wasn't likely that anyone would ever see it EXCEPT me, the builder. But knowing that I WOULD ALWAYS see it, I decided to eliminate it as best as I could. Fortunately, I was planning on painting the hull and therefore, I wasn't concerned with messing up the appearance of the wood grain.
I will describe what I went through and I must point out that, IMHO, doing this job on one boat does NOT make me an expert. But this is what I experienced:
I began with silica. Since I had never used these products before, I knew that this would be a learning experience. I had absolutely no problems with sagging. I have to admit that it never occurred to me that sagging was even a possibility. So, I slathered the stuff as thick as I could in the first pass. After it had cured, about 24 hours as I recall, I sanded it enough to break the surface, did quite a bit of eyeballing and repeated the entire process. The silica was difficult to sand for me, especially at first. You really need to break the surface before the sanding gets easier. I used my trusty air file and, even then, I was working it hard enough that the air file had my compressor down on its knees to keep up. And my compressor is a large commercial 220v job.
It took several applications before I started getting it where I wanted it. This is a natural occurrence because you always sand off most of what you have just put down. It doesn't really work if you try to save work by short cutting the sanding effort. So I was thinking in terms of doing as many applications as I felt were necessary to get it where I wanted it.
When I began to come really close to getting the results I wanted, I switched the final applications to the micro balloons because I was thinking that they would sand a little smoother and easier. They did. But I have to say that my use of the word "easier" is relative. Neither product gave up its initial shape easily. In the end, it all came out as close to perfect as I could have wanted and so, I'm very happy. But WHEW!! It was a lot of work even with the air tools. If you don't have air tools AND you DO have a large area to fair, consider fabricating a large two-man sanding board. You can use sandpaper made for the older style drum sanders used for hardwood floors. It comes in rolls and this means that you can cut the paper to fit whatever size sanding board you're using. You will have to find a supplier for the professional hardwood flooring industry to find that paper. It is somewhat expensive going in but it's an investment that will pay off if you plan on building more boats in the future. On the other hand, If you only have small areas to fair, then a one-man sanding board will work fine. For that, I use the type of sand paper that body shop guys use. I've gotten excellent results with both types of paper.
When fairing this stuff, be sure to use an X pattern when sanding. I have always described this as sanding back and forth from east to west for a while and then switching from north to south for a while. This is the best way to get your work even with the surrounding area. Run your hands over it to FEEL the irregularities. You can't always see them during the sanding process but, with a little practice, you can definitely feel any high and low places that still need work. Because I knew that I was going to paint my topsides anyway, I used different colors of spray primer as guide coats. I would sand until I thought things looked good and they felt good when I ran my hands over the work area. Then I would spray a LIGHT coat from ...say... my red oxide primer, let it dry and then sand it again. The primer will sand off of the high spots and will stay in the low spots. After that, if I needed to spray another guide coat, I would switch to ...say.. my gray oxide primer. That way, I wouldn't be confusing one pass with a previous pass. This is a really good way of seeing how far you still need to go.
I hope I haven't talked too much.