Royal Palm Resin is an epoxy resin. It comes in 2 types a thin and an original formula. The thin formula is very fluid (a bit thicker than water) and the original is a more viscous type (more like thick syrup or honey).
There are 3 main types of resin used for casting by home users.
Polyester Resin often referred to as PR. The most commonly used is Silmar41 or other similar brand names available at hobby, craft, hardware or online stores.
Epoxy Resin (this is what Royal Palm Resin is). It comes in a number of formulas with different characteristics including clarity, set time, UV resistance etc. Many epoxies are not suitable for casting.
Polyurethane this comes in a number of variations with different characteristics.
Of the three types of resin typically used for casting PR (polyester resin), Epoxy, and PU (polyurethane) each has it's uses and it's pro's and cons I will try to do a broad overview of those.
Pros - It is the least expensive resin, it can be cast without a pressure pot, it produces nice clear casts, it is readily available even locally.
Cons - It smells very bad, it is very toxic and requires very good ventilation, it can be fussy about temperature and humidity, it can act as a solvent for a number of adhesives, paints, and dyes which you may be using in your casting.
Pros - It is a produces nice stable casts, it cures very quickly, it works well with many dyes and pigments.
Cons - Mixing needs to be very precise especially on smaller castings and is mixed by weight, it has a very short working time, it requires a pressure pot, it does not react well with CA glue (super glue often used with casting), it is very sensitive to moisture and will cloud if it comes in contact with it, short shelf life, price vs PR
Pros - Not reactive with most materials, doesn't cloud with moisture, long open time, long shelf life, nice clear casts, bonds well, can be cast without a pressure pot.
Cons - Longer cure time, price vs PR, prefers to be cast in warmer temperatures.
Yes you can cast Royal Palm Resin with no pressure pot for many applications if you follow the instructions provided. With that said you will always see the best results when using a pressure pot with Royal Palm Resin or any casting resin and it is advised to use when appropriate.