Thin? Thick? It's all relative. You should know the viscosity of the resin you are working with. Most people cannot tell the difference between a 10cps resin and a 20cps resin and both would be considered thin. It's important to remember that viscosity is not just how thick or thin a liquid is, but it's ability to flow and anything under 20cps will flow out into the ends of the smallest of cracks with the proper equipment and technique.
Problems filling stars is sometimes the equipment and sometimes the resin, but more often than not it is the technique and/or the skill of the technician. If you haven't had mfg training on the equipment you purchased I highly recommend it, although I often read posts on this forum from Delta Kits trained technicians that make me cringe. Some are obviously not following our recommended practices and therefore continue to struggle with repairs that would not be a problem if they were using the equipment properly.
It's also important to remember that getting resin to the end of the cracks is only one part of the equation. The resin also has to have exceptional bonding strength and flexibility in order to handle the extreme heat and rapid heating and cooling that occurs when washing a vehicle or running the A/C in hot weather.
You should not have to drill the end of the short cracks found in a typical star break, regardless of your climate. If you are using a high quality resin and getting it to the ends of the cracks, the repair should be permanent.