I don't know if interfacing is considered as part of sewing notions.. but never mind, I like it this way so that my posts are organised nicely.
For newbie on the journey of sewing:
1) What is interfacing and the different types of interfacing?
Interfacing is either fusible or non-fusible and it is either woven, non-woven, or knit.
It is an extra layer of fabric that provides shape, support, stiffness and reinforcement.
Read more at:
http://www.fabriclandwest.com/Notion_basics/Interfacing/interfacing.htm#options

I figured the interfacing I normally bought from the shop here is called fusible non woven interfacing. They are either light, medium or heavy weight. So far, I have only tried light (RM2.0 per meter) & medium weight (RM2.2 per meter).
It is called Kertas Gam but actually it is not made of Kertas! Before this morning, I thought it is really "kertas yang disapu dengan gam". hehhe.. I even thought that the gum will dissolve in water so it should stay dry before use though I wonder what happen when we wash the cloths which use "Kertas Gam". I was ignorant..
I soaked a small piece of the interfacing I have and it is still strong when it is wet and of course no gum came out. :)
I have also bought non fusible non woven heavy weight interfacing before, used for collars. I think this is "Kertas Keras", if I am wrong please let me know. I have cut a strip for my yet to sew Balut (form part of the waistline of the long skirt of Baju Kurung). This is also available in long strip at the shop sold by meter. But to save cost, I cut it from my big piece of Kertas Keras.
But then, these are only 2 types of interfacing material. If you read the above
link, there are many types.
At the shop, you will see the different weight of interfacing rolls standing at one corner of the shop.
2) How do you choose which interfacing to use?
http://www.fabriclandwest.com/Notion_basics/Interfacing/interfacing.htm#choosing
As a general rule of thumb, choose one which is lighter than the fabric and with similar care requirement. Beware that fusible interfacing is not for all fabrics.
Here is a nice chart that categorises the fabrics into weight categories and the type of interfacing to use:
http://www.fabriclandwest.com/Notion_basics/Interfacing/interfacing_chart.htm
3) How to apply fusible interfacing?
In the past, I happily slide the iron on the interfacing directly.. erhh... this morning I learnt that:
(i) Steam the interfacing first to preshrink it. Assuming you have an iron with steam coming out :) , hold it over the interfacing not touching it for a few seconds. If you don't have steam iron or you are very hardworking, then prewash, please see here:
http://sewing.lifetips.com/tip/69341/fabric/interfacing/preshrink-fusible-interfacing.html.
(ii) Use a press cloth and spray water to dampen it evenly (not soaking wet). Put the press cloth on top of the interfacing, use
a little higher heat than what is required by the fabric. Apply pressure for about 10 seconds for light weight and slightly longer for heavier weight. Other than heat, one other key success factor is pressure. So, if your ironing board is too high, this can be difficult and tiring, lower down your ironing board.
(iii) DON'T slide, press and lift... aiyoh should have learnt this long ago, no wonder my piece of fabric that I ironed the interfacing on, become bigger as it was stretched and glued onto the interfacing.. :(
(iv) Baste the interfacing and fabric by touching it in several places with the tip of your iron. This will bond the interfacing to your fabric lightly so that it won't shift.
BTW, the shining side is the side which will stick.
(v) Patience and don't lift your fabric piece before it is completely cooled, otherwise the bonding process maybe affected.
(vi) For an expensive piece of fabric, better test it with a piece of scrap to avoid an expensive mistake. :)
(vii) If you don't have a big enough piece, the smaller pieces of interfacing can be joined and used as a big piece. See
link for instruction under the "More Tips" section.
(viii) Store them in a seal plastic bag. I am guessing that this is becasue they attract dust easily...
For more details, here is a good link:
http://www.sewingsupport.com/how-to-apply-fusible-interfacing.html
For making beg/purse/wallet:
1) Please refer to
http://anic-lohas.blogspot.com/2010/07/wallet-project-part-2-looking-for-tips.html#tipforinterfacing for recommendation of cutting & ironing interfacing.
2) Interfacing Guide from Amy Butler
http://sewmamasew.com/blog2/?p=154
I have done this research on request of Meena (Zara). I hope this meet her needs. I thank her for making this request, gave me an opportunity to clarify quite a few doubts I have about interfacing.
Thanks to the owners of the above links for sharing this useful information.
Happy experimenting with your interfacing materials...