Experiment to find what is best for you and what is available depending on where you live. Following are a few tips and suggestions you can use use on felts or straws or fabric, but always experiment and test firstly. Please take care in handling all stiffeners, refer to manufacturer safety information for potential hazards and first aid measures.

1 - Shellac
Using shellac flakes which dissolve in alcohol (ethanol fire fuel is cheap, easy to get, and unlicensed (UK) - just use with ventilation). Dewaxed shellac is cheap too, and the only thing you can't use it on is anything very pale colours. You can find Shellac on eBay or websites that supply French polishing products. Buy dewaxed kind -the blonde comes in different shades. Choose from the palest to dark brown according to felt colour. Use a few teaspoons - minimum four, but depends how strong you want it & experiment adding 500ml of alcohol. Put the flakes in the bottle, pour the alcohol on, shake it a bit, leave it a few hours and shake it again.

Then strain it to get out any residue, and brush on. Brush it on the inside where you can like any stiffener - again do some testing out to make sure you're happy with it. The alcohol has to be at least 90%. Rubbing alcohol works, though you can't easily get that in the UK because of licensing laws. Hence ethanol fuel for fires or BBQ sell it in UK.
Best for: Felt

2 - Spray Starch
Use Spray Starch on a semi stiff hat. If you use PVA glue neat on a crown you will need to line it – never apply neat to brim as it leaves white surface
Best for: Felt

3 - Zapon
Zapon varnish for metal can also be used when diluted.
Best for: Straw, Fabric

4 - Gelatin stiffener
Gelatin stiffener in powder form from most suppliers works well and shipped internationally.
Best for: Felt

5 - Water-Based Stiffener
House of Adorn Stiffener non toxic - water-based stiffener This water-based hat stiffener is made in the UK and has been supplied to the Millinery industry since the 1980's. It is made with a PVA base and can be easily diluted with water (normally 1 part stiffener to 4-5 parts water) and applied with a spray bottle, brush or immersed in a bucket/tray. It is non-flammable and non-toxic so it is safe to use and easily transported. This stiffener is mainly used in Millinery but can be used on other fabrics as well.

It is highly recommended that a small sample/area is tested first before applying stiffener to the entire area to be treated. If spraying, start with a light spray and continue to spray layers until desired stiffness is achieved. If the stiffener is applied too densely or on dark colours, it may result in a visible 'white/cloudy' residue (so always remember to test first before use). For those who want to stiffen thicker materials such as wool felt, you may want to consider the Solvent-base Hat Stiffener which is stronger but is toxic and extra care must be taken when using it. 
Best for: Straw, Felt, Fabric

6 - Hydrolac Powder 
Leko USA Supplier suggests :- Using hydrolac powder for hat stiffening instructions for one hat.

1. Using a plastic “throw away bowl and spoon”, while stirring, add 1/2 of the package (or 1/3 cup) of the HYDROLAC Powder to 1 cup of isopropyl alcohol (70%) and 1 tablespoon of water
2. Stir until lumps are gone. Mixture will be thick like oatmeal or wallpaper paste
3. Cover with plastic wrap. Allow thick slurry to stand for one hour in a safe place and away from children and open flames. Mixture will become clear and liquid
4. Pour mixture into small spray bottle
5. Spray the under side of the brim lightly and use a bristle or foam brush to smooth the HYDROLAC into the felt
6. Allow the brim to dry. Re-apply if necessary
8. Set with steam. You can use an iron or pot of steaming water to steam the hat to SET the HYDROLAC

Always test some extra felt with the hydrolac and let it set for a while in a warm place. you can check to see if there is any color distortion with the lighter colors. if you use alcohol on highly dyed fiber that has not been rinsed well during the dyeing process, colors may bleed or be altered slightly. If you decide that your hat is too stiff, saturate the area with alcohol and soak up with paper toweling. Reshape and let dry. Remember to reset the hat with steam.
Best for: Felt

7 - Solvent based Stiffener
Use Hat Stiffener purchased from Millinery Suppliers - buy the thinner as well which dilutes the stiffener from the brush. Pour an inch or so into jam jars with tight lids - Stiffener in one jar, Thinner in another (eg. jam jars with tight lids) - Label each jar Stiffener and Thinner. When finished stiffening, dip brush in thinner jar and drench brush, wiping excess on side of jar - the brush is ready for next time. For under brim use mix of 2/3rds stiffener to 1/3rd thinners. Can use straight on inside crown but if after you have blocked brim apply 2 light coats on UNDERNEATH of brim before you wire. Make sure it is dry after steaming before you stiffen & use in open air with ventilated mask - toxic to lungs and brain.
Best for: Straw, Fabric, Felt

8 - Fabric Stiffener for flower making
1. Bring to boil 420ml of water in a saucepan
2. In a cup mix together until a thin paste - 20ml of pure corn flour/corn starch with 20ml of water (not cheap Wheaten Cornflour)
3. Very slowly and stirring constantly pour this paste in to boiling water – no lumps . If there are lumps take off stove and mix vigorously
4. Return to stove and allow to boil for 3 mins until clear and thick. Remove from stove
5. Measure out into cup 40ml of PVA glue and hold high above saucepan and trickle in slowly then stir in quickly with starch mixture while it is hot Ensure all is smoothly combined
6. Pour into glass jar & cool before use. Keep sealed and it will last 7-10 days
7. Use a paint brush to apply to fabric evenly. Use large embroidery hoop to stretch silk flat is ideal or hang on clothes airer and use once it is dry & stiff
Best for: Fabric 

09 - There are a few felt stiffeners on the market. 
Bickmore Kahl hat stiffener lasts longer than the other brands, like Scout. Bickmore is nothing more than denatured alcohol, and a natural resinous glaze, it seems to last longest on hats that are a bit 'tired'. Other brands work just as good as a clean hairspray. A good way to go to save a few bucks. Then you just need to freshen it up now and then, and hairspray is about $2.00 a can as opposed to $5 to $9 for a bottle of stiffener. As always, steam first to get your shape. Material Safety Data Sheet.
Best for: Felt, Straw

10 - Handy tips to stiffen felts

11 - How to Make My Own Felt Hat Stiffener

» Hat Stiffener Discussions

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