How many times has someone said to me "I don't know how you do that so easily!" This is after they have watched me applying a line of mastic around a newly installed door or window frame. The simple answer is --practice, and practice! The same statement could be applied to a plumber who smears his or hers flux and solder to a joint to make a leak free joint or a brickie or plasterer who effortlessly appear to apply their trade as though by magic. These tradespeople do not acquire their skills overnight. It can take many years for a master plasterer to gain the skills that will give him a living, or for a window fitter to finish the job with a nice neat line of mastic to provide a weather proof protection from the elements.
Applying silicone mastic successfully can depend on so many criteria. The brick or render surface next to your frame may be ragged and uneven, in which case you will have to work harder to achieve a neat finish. The dream scenario is a smooth and straight surface that your mastic gun will glide over and leave a perfect line twixt frame and wall that needs no further attention. They always say that you should start from the bottom and work your way upwards gently squeezing the trigger of your mastic gun so that the mastic leads into the joint before the nozzle , filling it enough and self smoothing it as it goes without too much excess and slowing down on the trigger pressure as it reaches its end cycle. Of course the perfect line of mastic first time is hardly ever achievable! Most professional window fitters will find it irresistable to not run a moistened finger down the line to iron out any imperfections and back again in the opposite direction to counter any drag lines. Of course for health and safety reasons your D.I.Y.ers should avoid any skin contact with silicone as it is supposed to be a carcinogenic material and may possibly do you harm. There are devices on the market that you can purchase that look very similar to an index finger, or you could make your own but sad to say I still prefer 'the finger' but that is the risk I personally take and may live to regret it!
Obviously the cut size of the end of the nozzle on the end of your tube of mastic must be a tad larger than the gap that you wish to fill. Try to cut the nozzle cleanly with a sharp craft knife as any raggedness will affect the finished line. Gently squeeze the mastic guns trigger and push the gun before you lightly in an upwards direction barely making contact between the frame and brick/plaster work, and keep a steady pressure on the trigger. There is no hurry! You can of course reverse the way you hold your gun and apply the mastic as you drag the line behind the nozzle from top to bottom for instance, this may require a little more practice, but can still be very effective and is the way I often prefer. On a basic skeleton gun be ready to push the overrun tab to stop any unwanted flow of mastic as you reach the end of the mastic line. Some guns do this automatically when you release the trigger pressure.
Low modulus silicone is the trade standard for exterior applications nowadays and decorators caulk is for inside use as it can be painted over with emulsion or oil paints as it is a water based medium unlike silicone that resists most paints and varnishes,.
Remember that most modern mastic's start to 'go off' fairly quickly, so do not leave it too long before you tidy it up wiping any excess off with a cloth or smoothing as described above. Mastic's come in many colours from clear to white, toffee, brown and black, so choose the colour you think will complement your installation best and match the frame colour to suit.
I do hope you have found the above of some use and if it helps you to perform a satisfactory finish then that is all we can hope for without the years of practice that can make perfect.
Please be so kind as to tick one of the boxes below if you think this advice could be helpful to you. Good luck and practice a bit of your skill on the inside edges of a cardboard box first or somewhere behind the garden shed if it helps to get the feel of mastic and gun. Its fun, not rocket science!---Trev.